For conditioning to occur, a(n) must repeatedly come before a(n) unconditioned stimulus.

The correct answer is a neutral stimulus. 

A neutral stimulus (such as a bell, or an object), that is repeatedly followed by unconditioned stimulus, eventually elicits a conditioned response. For instance, in Ivan Pavlov's famous classical conditioning study, he repeatedly paired a neutral stimulus (a ringing bell) that came before an unconditioned stimulus (food), and eventually the sound of the bell alone elicited a conditioned response (salivating) in dogs. 

Iwould say the answer would be b