Hi again Paula;
The rabbits live in the yard but are difficult to find.
The rabbits live in the yard ...This is an independent clause.
but ...This is a coordinator. It is one of several characteristic of compound sentences.
are difficult to find ...This is neither a dependent nor independent clause. It does not have a subject.
This is a simple sentence. The answer is C.
For this to be a compound sentence, it would possibly be...
The rabbits live in the yard but these animals are difficult to find.