Answer: C) because their offspring are infertile
Step-by-step explanation:
A species can be define as the group of members which are capable of inbreeding and producing viable fertile offspring. The members of two distinct species can inbreeding but they cannot produce viable fertile offspring. The offspring will be infertile.
According to the given situation, the horses and donkeys are considered as different species even though they can inbreed as they are incapable of producing fertile offspring.