The approach that contends psychology must study internal mental events in order to fully understand behavior is:

Cognitive psychology is the study of internal mental events to fully understand behavior.

Explanation:

Cognitive psychologist's interest in how people handle the problem. It examines the internal mental process of the people in analyzing and solving the problems or issues.

These psychologists mainly concentrate on how people react to the problems and analyze what steps they were taken to come out of the problem. There were no framed rules to be followed to identify the issues in the people.