Scientists think that two species with homologous structures probably had a a. environment b. offspring c. fossil d. ancester

D) ancestor     
for example, apes and humans have some genes and genomes that are the same. we are believed to have come from a common ancestor because of not only that, but the structures of our bones are very similar to one another's. 

D.) domain, kingdom, phylum, class, order, family, genus, species