Scientists combine evidence from fossils, body structures, early development, dna, and protein structures to

a.

determine what bones an animal has in its forelimbs.

b.

determine the evolutionary relationships among species.

c.

decide which fossils are older than others.

d.

determine whether an organism will have gills during its early development.

b.  determine the evolutionary relationships among species.  

Explanation:

Phylogenetics is the branch of science which deals with the determination of evolutionary relationship among the organisms. This branch of study derives relationship on the basis of similarities between the ancestors and their descendants. A phylogenetic tree can be constructed on the basis of such similarities.

Scientists try to find the similarities and differences among the organisms which used to exist in the past with the present species. This is based upon evidences that are obtained from the fossils, morphology or physical structures, type of genetic material present (DNA or RNA), protein composition an  features of early development.