Answer:
A. Complex
Explanation:
A complex sentence is a type of sentence made up of an independent clause (a clause that expresses a complete thought) and a dependent or subordinate clause (a clause that can not stand alone as a sentence because it does not express a complete thought). In this type of sentence, the dependent clause is often introduced by a coordinating conjunction such as when (like in the sentence given), after, while, because, since, if and until, which also indicate the subordination of the clause to the independent clause.
The sentence given is structured like a complex sentence: It has a dependent clause, which it's When Sara turned her homework in and a dependent clause: she forgot to put her name on it.
