Sloths have less muscle than other mammals their size in order to
a. conserve energy
b. maintain a lower body weight
c. digest their food faster
d. maintain a higher body temperature

The specific adaptation of the sloths to their niche is energy conservation: they survive by trying not to loose any energy and trying not to move too much. They have less muscle because they don't need it: their adaptation is to move less. So the correct answer is to a) conserve energy.