The correct answer is B. simple.
A simple sentence contains only one finite verb - and if you take a look at the sentence above, you can see there is only one verb: IS. This means that it is a simple sentence.
A complex sentence contains one independent clause and at least one dependent clause. A compound sentence contains at least two independent clauses. A run-on sentence uses commas incorrectly in order to connect two or more independent clauses, which is not the case here.
