The forelimb of the monkey, bat, penguin, and alligator look
very different on the outside, but their skeletal structures are
similar. this evidence suggests that these animals

The given examples shows homologous structures and divergent evolution.

Explanation:Penguins, alligators, humans, monkey and bats are very different creatures.  Nonetheless, they all have very similar structures. Each have a humerus, radius, ulna, and carpels in their forelimbs.  Although each bone looks different in each species, the bones serve similar functions in the different organisms.  This shows homologous structures.Homologous Structure: A homologous structure is an example of an organ or bone that appears in different animals, which are anatomically similar structure demonstrating descent from a common ancestor.This shows divergent evolution as they have evolved through common ancestor.

Result: The given examples shows that the animals have homologous structures.  

I’d say a : water molecules