Dogs and cats filled the animal shelter and the noise they made was incredibly loud.
Considering the underlined phrase, C. animal is the correct answer. Animal is a noun but it has got an adjectival function here; it has been placed before a noun: shelter , so it tells what sort of shelter the speaker is refering to. For example, a bus shelter is the place where passangers wait for the bus. Bus is also a noun functioning as an adjective.
These options are wrong:
A. shelter: This is a noun functioning as a noun.
B. noise: This is a noun functioning as a noun.
D. they: This is a pronoun, so it substitutes for animals.