If mitotic cell division is the only way a particular species of single-celled organism can reproduce, it is most likely that

If mitotic cell division is the only way a particular species of single-celled organism can reproduce, it is most likely that the rate of evolution in this species is slower than in one that reproduces sexually. This is asexual reproduction, which causes mutations in a population to accumulate and combine more slowly than sexual reproduction in a cell line, as the mutations would have to arise multiple times in one line, rather than a few separate times and then be allowed to combine sexually for the descendants to have those traits. 

The answer is option a