Is the sentence simple, compound, or complex?

as he returned to his village, people stared at him strangely.

a.
compound

b.
complex

c.
simple

The answer would be B, complex.
Compound is two complete sentences that is combined by a coordinating conjunction such as but, for, and, etc. For example, "I waited for Lisa at the bus station, BUT the bus didn't arrive until noon and so I left.
 Simple means that a sentence uses just a subject and a verb. For example, "The bus was late" subject=bus; verb=was
Complex is a sentence that has a independent clause and one or more dependent clause. The dependent clause starts with a subordinating conjunctions which are as, after, because, etc.
In this particular sentence, it starts with As and, it has a independent clause which is "He returned to his village"