Which of the following is a theory of forgetting?

The Interference Theory of forgetting holds that we forget not because a memory trace simply fades away, but because competing information gets in the way of storing or retrieving it. Two directions of interference are recognised: proactive inhibition, where earlier learning disrupts new learning, and retroactive inhibition, where newly acquired learning disrupts the recall of material learned earlier.

This question asks which of the four listed processes is an actual, recognised theory or mechanism of forgetting. Among the four, only retroactive inhibition names a real process within Interference Theory: when a person learns new material after learning something else, that new learning interferes with retrieving the earlier material, producing forgetting of the earlier material.

The remaining options are not established forgetting mechanisms:

• Competitive inhibition is a term from biochemistry and enzyme kinetics, describing how a competing molecule blocks a substrate from an enzyme's active site — it is not a psychological theory of forgetting.

• Instructed inhibition is not a recognised process or theory of forgetting in psychology; no established mechanism carries this name.

• Reflected inhibition is likewise not a recognised process or theory of forgetting; the term does not appear in interference-theory literature.

Only retroactive inhibition names an established forgetting mechanism under Interference Theory, which is why it answers the question.