Answer: C) retroactive interference
Explanation: Retroactive interference can be simply defined as a situation where recently encoded memories gets in the way of previous encoded memories during recalling, i.e recent information stored in our memories gets in the way when we try to recall previously stored information.
Keith's forgetfulness of his sociology material and the rememberance of only his psychology material when he tries to recall the sociology material he studied previously best illustrates how recently encoded memories interferes with previous ones during recalling. This is called retroactive interference.