Ivan pavlov introduced the concept of when he taught a dog to salivate at the sound of a tuning fork.
a. reinforcement
b. unconsciousness
c. conditioning
d. reward

Ivan Pavlov, the famous Russian physiologist, introduced the concept of conditioning when he taught the dog to salivate at the sound of a tuning fork - c.

The reason why the dog was salivating was because previously he gave the dog an immediate reward such as food together with the tuning fork. He then gradually changed this and gave him food on more rare occassions while stile preserving the frequency of the tuning fork.