Question

select the words that make up the complete compound subject. black rat snakes and timber rattlers often hibernate in dens together.

Explanation:

A compound subject is made up of two or more simple subjects that share the same verb. In the sentence "Black rat snakes and timber rattlers often hibernate in dens together", "Black rat snakes" and "timber rattlers" are the two parts of the compound subject.

Answer:

Black rat snakes, timber rattlers