Pavlov's dog Rex drooled heavily at the sight of a blue dog food bowl. Pavlov used extinction to stop the salivating. Two weeks later, however, Rex began to salivate again at the sight of the blue bowl. This is an example of:

Spontaneous recovery

Explanation:

Spontaneous recovery is a term associated with learning and conditioning. Specifically, spontaneous recovery is the reappearance of a response (a Conditioned Response; CR) that had been extinguished. The recovery can occur after a period of non-exposure to the Conditioned Stimulus (CS).