Question

each of these relationships reflects a correlation. which relationship most likely reflects correlation but not causation?
riding a bicycle more often is associated with hiking more often.
riding a bicycle more often is associated with stronger leg muscles.
riding a bicycle more often is associated with wearing a helmet more often.

Explanation:

Causation implies one variable directly causes the other. Riding a bicycle more often doesn't directly cause more hiking. It's just a correlation. Riding a bicycle more can cause stronger leg - muscles (causation). Wearing a helmet more often is a safety measure related to riding a bicycle more often and can be seen as a result of more cycling (causation in a sense of safety - related behavior).

Answer:

Riding a bicycle more often is associated with hiking more often.