Why are tumor suppressors usually recessive? a. because by definition a mutant protein

The correct answer is option b. "Because a loss of function mutation of only one allele usually has no effect on the phenotype".

Explanation:

A recessive allele, does not have an effect in a phenotype if a dominant allele is present, it should be present in the two alleles in order to be expressed in the phenotype. A loss of function mutation of only one allele usually has no effect on the phenotype. This is the case for most tumor suppressors, which unfortunately most of them are recessive while mutant oncogene alleles are typically dominant.

c) Having a small population size

Explanation:

Endangered species are generally those whose population is declining or in a very limited range. Some of other characteristics of endangered species may also be habitat loss, indiscriminate hunting by man or predators, and disease.