Which theories of conditioning subscribe to the idea of "spontaneous recovery"?
Both classical conditioning and operant conditioning subscribe to the idea of spontaneous recovery to some degree. However, the phenomenon is generally referred to as "resurgence" in operant conditioning.
Spontaneous recovery is something that can occur in classical conditioning when the conditioning has been stopped for a period of time. The behavior that the subject learned through the conditioning may come back on its own (it may be spontaneously recovered) even after the conditioning has stopped. So, in the example of Pavlov's dogs, this could happen after Pavlov stopped giving the dogs food when the bell rang. If, after a long time of him not giving them food, the dogs started salivating again when the bell rang, that would be spontaneous recovery.