Dr. hardy is studying the correlation between physical exercise and health in old age. which is the most likely representative sample?

I found the question on the internet with the following options:
"A) elderly marathon runners
B) elderly people who just had hip replacements
C) a random group of elderly people
D) people over age 65 who work out regularly"

The correct answer would be option C).
A representative sample must be indeed representative of the widest variety of people there is for that study in particular. If the objective of the study was to correlate physical exercise and health in old age, the group should not be composed of older people on a great physical shape. Other wise the improvement on their health would probably be very discreet as they are already well physically exercised. This is the case of options A) and D).
On the other hand, option B), a group made only of people in physical recovery, or under any type of physical limitation, would condition the results as this group would be very limited as to the degree of exercise they could preform.

Could you elaborate?