Explanation:

Bag of bones (idiom):
an emaciated person or animal; weak and thin

It is used for a person or animal that is emaciated to the point where one can see bones protruding from the skin. (The skin is the figurative bag.)
It is a disparaging term for a very old person, especially one who has a frail appearance.

Ex: The old, abandoned dog looked like a bag of bones after living on the streets for so long.
Poor Dave, he looked like a bag of bones when he was done with his cancer treatment.

Hence, option A is correct.