Final answer:
The correct answer is 'case study'. In a case study, an in-depth investigation is conducted on a specific individual or group to gather detailed information and draw conclusions.
Step-by-step explanation:
The correct answer is b. case study.
A case study is a research approach where an in-depth investigation is conducted on a specific individual or group. Sigmund Freud used this method to develop his theory of human personality by conducting extensive interviews with a few clients over an extended period of time.
This approach allowed Freud to gather detailed and comprehensive information about his clients, which helped him draw conclusions about human psychology.