Obesity is a common nutritional concern during adolescence. the availability of fast foods is a reason for obesity because

The availability of fast food is a reason for obesity becuase IT DELIVERS LARGE PORTIONS OF HIGH CALORIES AND HIGH FAT FOOD AT LOW PRICES.
Fast foods usually consist of high carbohydrate contents and high fat content. These types of macro nutrients are usually stored in the body when they are in excess. Thus, frequent consumption of these type of food can easily lead to the development of obesity.