A compound sentence combines two complete independent clauses together. Compound sentences should be punctuated either a semicolon between the ideas or a comma and conjunction. In the correct sentence, the conjunction, but, joins the two complete ideas "Belinda called the office" and "no one answered" together.
The only other sentence that seems to have two complete ideas is the last option. This is a complex sentence. Using the conjunction "because", changes the sentence "she was running late" into a dependent clause. A complex sentence contains and dependent clause and an independent clause.
