Final answer:
The correct answer is option b. The most suitable method for acquiring detailed information about people's purchase habits is in-depth interviews, as they provide rich qualitative data and insights into personal histories and motivations behind behaviors.
Step-by-step explanation:
To acquire detailed private information about people's purchase habits, including past history, the most suitable research method would likely be B) In-depth interviews. This approach allows for a comprehensive understanding of the individual's experiences and behaviors by collecting nuanced information that may not be captured in a survey or observational research. In-depth interviews are similar to the case study approach used by Sigmund Freud in developing his theories.
While other methods like surveys and focus groups can collect data from larger samples, they may not provide the depth of personal and historical information that can be obtained through in-depth interviews. Additionally, observational research may not reveal private motivations behind purchase habits. Longitudinal research could track purchase habits over time but may not delve as deeply into the personal history behind those habits.
For studying sensitive topics where a high level of detail and context is needed, in-depth interviews offer a level of depth unmatched by surveys, focus groups, or observational studies. This makes them a highly effective tool for researchers in social sciences, such as sociologists or anthropologists, who require rich qualitative data.