Darwin's The Descent of Man differed from On the Origin of Species in that:

a. It was based on close observation of one particular species of tortoise.
b. It was less well-written and received poorly by the reading public.
c. It dealt with the subject of human evolution.
d. It was published before Darwin had become a familiar author.

c. It dealt with the subject of human evolution.

Explanation:

The origin of the species tells about the evolution of the species through the natural selection and the descent of man is the evolution of man as a single specie which began on the African continent.  And this theory focused on the sexual selection and process of the human evolution that are closely connected and he found that the selection of males is different from those of the females.