A complex sentence is a sentence which has an independent clause and one or more dependent clause. An independent clause is which can stand on its own as a sentence but a dependent clause cannot stand on its own, it need an independent clause for it to sound meaningful as a sentence. A complex sentence needs to have both dependent and independent clauses.
In the given question, the complex sentence is option a) I will work for you if you pay me good. The clause “I will work for you” is independent and “if you pay me good” is dependent on the independent.