Step 1: Understanding the Concept:
Psychologists use various methods to investigate human behavior. These range from controlled experiments to narrative observations.
A Case-study is a qualitative research method that involves a detailed, comprehensive, and longitudinal investigation of a single unit—usually a person, but sometimes a group or institution.
It is "idiographic" in nature, meaning it focuses on the unique qualities of the individual rather than general laws.
Step 2: Detailed Explanation:
The prompt uses several keywords that point directly to the Case-study method:
1. "In-depth study": Unlike surveys or tests which capture a "snapshot" of a person, a case-study dives deep into every aspect of their life.
2. "Psychological history": This method tracks the development of the individual over time, looking for past events that explain current behavior.
3. "Context of psycho-social and physical environment": A case-study does not look at the person in isolation. It examines their family, work, culture, and physical surroundings to understand their reality.
Analysis of distractors:
(A) Interview: This is a technique of asking questions. It is often {part} of a case-study, but it is not the study itself.
(B) Self-report: This involves the person rating themselves on a scale or questionnaire. It lacks the "in-depth environmental context" provided by a case-study.
(C) Psychological test: These are standardized tools to measure a specific trait (like I.Q.). They do not provide the narrative "history" of the individual.
Step 3: Final Answer:
The description of a comprehensive, history-based investigation of a person within their environment is the formal definition of a Case-study.
This method is extensively used by clinical psychologists to understand the origins of psychological disorders.
Hence, the correct option is (D).