The best way to combine the independent and dependent clause to make it a complex sentence is as shown in sentence D. A complex sentence is a sentence that contains an independent clause and one or more dependent clauses. An independent clause, as opposed to a dependent one, can stand alone as a sentence as it represents a complete thought (see 1). (1) is not an independent clause as it fails to stand alone as a complete sentence, it depends on the main (dependent) clause to complete its meaning (see 2).
