Fundamental similarity due to descent from a common ancestor is called

fundamental similarity due to descent from a common ancestor is called homology.

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.

if you listed your answers right then i believe your answer is b.