Dogs and cats filled the animal shelter and the noise they made was incredibly loud.
Is:
They
Explanation:
The reason behind this answer is that the pronoun is a concept or a word used in languages to replace the proper name of the subject or object /subjects or objects of the sentence. In our case we have dogs and cats, we can group them with "they". Therefore this is our correct answer. We could have used many more, but this was the most general as we were talking about them.
