Which of the following does not limit range of motion?
a. weak muscles
b. age
c. disease
d. bone structure

What doesn't limit the range of moton from the things which you've mentioned would be age. Age per se doesn't really limit the range of motion - while there are factors which negatively influence our range of motion and which are strongly correlated with age, it is hard to say that age itself would be the cause of a limited range of motion.

Ibelieve the answer is c.