3. Some scholars doubt that he ever lived.

simple sentence

compound sentence

complex sentence

compound-complex sentence

complex sentence

Explanation:

A complex sentence is one that contains an independent clause and a dependent clause. The independent clause in this example is Some scholars doubt and the dependent clause is that he ever lived.

The dependent clause begins with the word that and cannot stand alone as a sentence. Were the sentence to have two independent clauses it would be considered a compound sentence, and it it had two independent clauses along with a dependent clause it would be a compound complex-sentence.