"Many animal species are totally colorblind, but the condition is very rare in humans".
The section is all about wrong choice of singular/plural, so I assume the question setter thinks there's a problem with "many animal species". There isn't: the singular and plural of "species" is "species".
The choice of colo(u)rblind / colo(u)r-blind / colo(u)r blind is a matter of style, not correctness.