Final answer:
Poor nutrition and a lack of structured exercise poses the greatest risk for chronic illness to young adults.
Step-by-step explanation:
Poor nutrition and a lack of structured exercise is the lifestyle choice that poses the greatest risk for chronic illness to the young adult.
Unhealthy eating habits can lead to obesity, high blood pressure, and high blood glucose levels, which are major risk factors for cardiovascular diseases and type 2 diabetes. Lack of structured exercise also contributes to a sedentary lifestyle, which increases the risk of chronic diseases.
On the other hand, alcohol and tobacco use, ignoring seatbelt and helmet laws, and unprotected sex with multiple partners are risky behaviors that can have negative consequences, but they do not pose as great of a risk for chronic illness as poor nutrition and lack of exercise.