the last one; color of fur/feather.
explanations:
DNA would be a sure fire giveaway because if the species were related the DNA would look almost exactly the same.
fossils would show bone structure or what the organisms looked like.
early development isn’t at all something someone would jump to to find if the species were related. BUT if you were trying to figure out if a frog and a lion were related it would be clear that they weren’t by their early development. a frog comes from and egg, and a lion is born.
body structure can also be a sure fire giveaway because if you wanted to know for example if a lion and a frog were related you could tell they’re not just by the bone structure.
color of fur/feather will not tell you if two species are related because there are multiple organisms that are from different classes that are the same color. a cardinal is the same color as a lobster isn’t it?