Final answer:
Substituting fresh vegetables for French fries would be the most beneficial diet change for a pregnant teenager, as it provides essential nutrients and supports maintaining a healthy weight during pregnancy.
Step-by-step explanation:
The dietary changes that would be most beneficial for a pregnant teenager who typically eats a hamburger, French fries, and diet cola, would be to substitute fresh vegetables for French fries. Vegetables provide essential nutrients and vitamins which are crucial for both the mother and the developing fetus. Considering the need for a balanced diet during pregnancy, which should include extra calories, proteins, lipids, and especially important nutrients like folic acid, calcium, iron, and omega-3 fatty acids, adding fresh vegetables would significantly increase the intake of these essential nutrients. The consumption of vegetables would also assist in maintaining a healthy weight during pregnancy, as it provides essential nutrients without too many excess calories, which could contribute to obesity and related health issues for the baby later in life.