A baby's development is influenced by both heredity and _________.

Environment

You should notice the word both which adds to a list in a positive way. You look for sentence form and detect that the [subject] and [verb] are in their place. it is important to understand that heredity is a noun. Option A is eliminated since it repeats too much information. Option B is eliminated because it is an adverb. Option C is eliminated because it is similar to A. Leaving option D as the correct option.