Final answer:
The underlined part of the provided sentence is a main clause. It has a subject and verb and can stand alone as a complete sentence, categorizing it as a compound sentence since it's joined with other main clauses.
Step-by-step explanation:
The underlined part of the sentence "Trevor climbed into the airplane, ran his hands over the controls before he started the engine, and prepared for his first solo flight." is referred to as a main clause. A main clause is a part of a sentence that contains a subject and a verb, and can stand alone as a complete sentence. It expresses the main idea of the sentence. This can be contrasted with a subordinate clause, which begins with a subordinating conjunction and cannot stand alone as a full sentence. The sentence provided does not contain subordinate clauses, only main clauses and a list of actions, which makes it a compound sentence, not a compound-complex sentence which would include at least one subordinate clause.