Final answer:
Among the listed risk factors, not wearing a seat belt while driving can have the most immediate impact on health, potentially leading to severe consequences in the event of an accident.
Step-by-step explanation:
The risk factor that can have an immediate impact on a person's health is not wearing a seat belt while driving. Unlike the gradual effects of lifestyle choices such as eating too much junk food, not exercising, or smoking one cigarette, the consequence of not wearing a seat belt can be immediate and potentially fatal in the event of a car accident. By buckling up, you provide yourself with the best defense against injuries or worse outcomes during a collision.