Final answer:
The risk factor with the most immediate impact on a person's health among the options provided is not wearing a seatbelt while driving, due to the potential for immediate and serious injury or death in the event of a car accident.
Step-by-step explanation:
The question asks which of the following risk factors has the most immediate impact on a person's health, choosing from eating too much junk food, not exercising for at least 30 minutes a day, not wearing a seatbelt while driving, and smoking cigarettes. While eating junk food, lack of exercise, and smoking cigarettes are all significant risk factors for chronic diseases such as heart disease, stroke, and cancer, not wearing a seatbelt while driving can have the most immediate and potentially fatal consequences in case of a traffic collision.
Smoking cigarettes is a major risk factor for cancer and cardiovascular disease, and not exercising or poor diet contributes to a range of health issues. However, these are generally cumulative and lead to health problems over a longer period. On the other hand, the action of not wearing a seatbelt can result in immediate and severe harm in an accident, which is why it is a critical safety measure that has immediate implications on health and survival in automobile accidents.