Hello there!
"The rabbits live in the yard, but are difficult to find."
It is a compound sentence.
A compound sentence contains at least two independent, but connected clauses by a coordinating conjunction, and this sentence has them two.
In this case, the first independent clause is: The rabbits live in the yard + BUT ( conjunction that links the two clauses) + the second independent clause (they implicit because of the ellipsis) are difficult to find.
A) compound complex: this kind of sentence is made up of more than one sentence, and at least one of those sentence is complex. In this case, both clauses are complexes.
C)Simple: this is not a simple sentence because it has more than 1 clause and a conjunction.
D) Complex: this kind of sentence is made up of more than one sentence joined by a subordinating conjunction. The word "but" is a coordinating conjunction, not a subordinating one.
Correct answer: B.
Hope this helps,