Corrected version:

The eating habits of people, without a doubt, have significantly shifted in the past 20 years.

Explanation:

"Eating habits" is a plural subject, so "have" should be used instead of "has" to agree in number. 

Improved version:

The dietary preferences of individuals have undeniably undergone profound transformations over the past two decades.

Corrected version:

Notably, the fast-paced lifestyle has led people to eat more and more fast food at the expense of traditional meals. 

Explanation: 

The verb "led" should be followed by the base form of the verb "eat," and the article "the" before "traditional meals" is not necessary.

Improved version:

Significantly, the relentless pace of modern life has propelled a surge in fast-food consumption, often at the expense of traditional, nutritionally balanced meals. 

Corrected version:

There are only a few upsides to consuming fast-cooked meals.

Explanation: 

"Few" should be preceded by the indefinite article "a," and "upsides" should be used instead of "upsites." Additionally, "fast-cooked" should be hyphenated for clarity. 

Improved version:

Nevertheless, from my perspective, the detrimental consequences of excessive fast-food consumption far outweigh any potential advantages. 

Corrected version:

This sentence is already grammatically correct. 

Improved version:

One pivotal merit lies in the cost-effectiveness and widespread availability of fast-cooked meals, making them an appealing choice for individuals seeking swift and economical dining solutions. 

Corrected version:

Another advantage of consuming fast food is the vast range of variety available, enabling individuals to satisfy their specific cravings, which may be lacking in traditional meals.

Explanation: 

"Carvings" should be corrected to "cravings," and "may lack" should be changed to "may be lacking" for gram matical accuracy. 

Improved version:

Another notable perk associated with fast food is its extensive variety, allowing patrons to indulge their particular cravings, a facet frequently absent from conventional cuisine. 

Corrected version:

According to BBC reports, about 93% of the young generation, especially in developing countries such as the USA and Japan, prioritise eating fast food over traditional food, which is attributed to the vibrant taste of instant cooked meals. 

Explanation: 

"The" should be added before "USA," and "BBC reports" doesn't require an article. "Instead" should be changed to "instant," and the sentence structure should be adjusted for clarity.

Improved version:

According to reports from the BBC, an astonishing 93% of the younger generation, particularly in developing nations such as the USA and Japan, gravitate towards fast food due to the tantalising flavours and convenience of instant meals. 

Corrected version:

Despite several advantages, the disadvantages of instant meals are multifaceted. 

Explanation: 

"A" should be removed before "several" since "advantages" is a plural noun.

Improved version:

Notwithstanding several commendable attributes, the downsides of instant meals are multifaceted and demand close examination. 

Corrected version:

As the evolution of food has decreased the nutritional content of meals. In the contemporary landscape, the majority of meals consumed contain only 30% of the nutritional value found in traditional food, ultimately causing severe health problems. 

Explanation: 

"Declined" should be changed to "decreased" to convey the intended meaning. "Contains" should be changed to "contain" to agree in number with the subject "the majority of meals." 

Improved version:

As culinary evolution has led to a decline in the nutritional content of meals, the majority of dishes consumed today provide only 30% of the nutritional value present in traditional fare, thus giving rise to a multitude of severe health issues. 

Corrected version:

The most profound drawback is its potential to lead to weight gain and obesity.

Explanation: 

"Lead weight gain" should be changed to "lead to weight gain" to make the sentence grammatically correct.

Improved version:

The most compelling drawback is the potential of fast food to contribute significantly to weight gain and the epidemic of obesity. 

Corrected version:

The exacerbating factor is that obesity can invite a plethora of diseases. 

Explanation: 

"Pelothra" should be corrected to "plethora," and "inviting for" should be changed to "invite a" for grammatical accuracy.

Improved version:

Furthermore, obesity serves as a harbinger of a myriad of diseases, further exacerbating the health hazards attributable to fast-food consumption.  

Corrected version:

For instance, in city hospitals worldwide, reported obesity cases surged by 85% in the last six years, making the impact of fast food evident. 

Explanation: 

"City" should be lowercase, and "the" before "reported" is not necessary. Additionally, "the" before "last six years" can be omitted for smoother flow.

Improved version:

A case in point can be observed in city hospitals worldwide, where reported cases of obesity have surged by a staggering 85% in the past six years, underscoring the far-reaching ramifications of fast food on public health. 

Corrected version:

In conclusion, the upside of instant meals, such as accessibility and affordability, is overshadowed by its profound cons. 

Explanation: 

"Instead" should be corrected to "instant," and the verb "is" should agree in number with the singular subject "the upside." Additionally, "such as" should be added for clarity.

Improved version:

To summarise, while fast food does offer commendable traits such as accessibility and affordability, these advantages are eclipsed by its substantial drawbacks.