Skinner argued that psychologists must restrict their investigations exclusively to:

A) overt responses made by a subject.
B) sexual and aggressive conflicts.
C) future goals and aspirations.
D) lower and higher needs.

Skinner believed in behaviorism, which focuses on observable behavior rather than internal mental processes. Therefore, he argued that psychologists should restrict their investigations exclusively to overt responses made by a subject. Other options such as sexual and aggressive conflicts, future goals and aspirations, and lower and higher needs are not directly related to behavior and may involve internal mental processes.