All humans belong to a single race because

a. there are no physical differences between human groups

b. there are no anatomical differences between human groups

c. all humans can successfully reproduce with each other

d. racial classifications are rigid

The correct answer will be option-C.

Explanation:

The race is a term used to denote the group of people who share common ancestors and descended from it. The groups are based on the physical appearance and social qualities as a result of adaptation and natural selection which led to the variation in the species of humans.

Although humans show variation, they all belong to the same species and same genus - Homo sapiens. The sapiens share 99% DNA as well they can successfully reproduce with each other which shows that they have descended from a common ancestor.

Thus, Option-C is the correct answer.