A scientist is trying to determine if two species are closely related or if they

just have the same external features. What would you look for to make this

determination?

If  two similar looking species are able to produce fertile and viable offspring, then they belong to the same species.

Explanation:

According to the biological species concept , organisms are said to be of same species if they are able to interbreed and produce fertile and viable offspring.  

In case if two species are closely related on the basis of external features, then they must be allowed to mate. The offspring produced in the first generation mating must be allowed to mate again. If in the second generation offspring are produced, then it shall prove that the two similar looking organisms are able to produce viable and fertile offspring thereby establishing the fact that they belong to same species.  

the answer is d.) homologous; have mammary glands