Answer: It is a simple sentence.
Explanation:
Compound sentences contain a <em>FANBOYS</em> (for, and, nor, but, or, yet, so) coordinating conjunction following a comma. Since this sentence does not contain a <em>FANBOYS</em> conjunction after a comma, it is a simple sentence.
example compound sentence:
I was hungry, so I made myself a sandwich.
example simple sentence:
I made myself a sandwich because I was hungry.