Question

how can teens demonstrate good physical fitness to their families?
by keeping their rooms tidy
by flossing their teeth every day
by saying please and thank you
by taking the stairs instead of the elevator

Explanation:

Taking the stairs instead of the elevator is a physical activity that can contribute to good physical fitness. Saying please and thank you is related to good manners (not physical fitness). Flossing teeth is related to oral hygiene (not physical fitness). Keeping the room tidy is related to cleanliness (not physical fitness).

Answer:

by taking the stairs instead of the elevator