A general rule for creating a plural form of medical terminology is that a singular word ending in an -a will keep the -a and add

absolute strength refers to the maximum force exerted by a body irrespective of body size. relative strength on the other hand is the force exerted by a body in relation to muscle size or body weight.

so the answer is: c. absolute strength measures strength regardless of your body size, while relative strength measures strength adjusted for your weight.

the answer the third choice the letter c