Which best identifies the structure of the sentence the rabbits live in the yard but are difficult to find is it a simple be compound see compound complex d complex?

The correct answer is compound sentence.
A compound sentence contains at least two independent clauses, which is the case here: the first independent clause is The rabbits live in the yard + BUT + the second independent clause (they) are difficult to find. 
A simple sentence would have only one independent clause; a complex sentence would have at least one dependent clause (apart from one independent); a complex sentence would have at least two dependent clauses (apart from one independent).