Final answer:
A case study is a deep investigative approach focusing on a single individual or a few people, which provides rich insight but has limitations in generalizing findings to a broader population.
Step-by-step explanation:
An in-depth focus on a single individual best describes d. case study. A case study is employed by researchers who wish to gain a deep understanding of a particular phenomenon by studying one person or a very small number of individuals in great detail. This often involves a combination of methods such as direct observation, interviews, and review of archival records. Developmental psychologists, for instance, may use case studies to track physical and psychological changes over the lifespan of an individual, and these studies often provide rich, qualitative insights.
However, a major limitation of case studies is c. the inability to generalize the findings to a larger population. Since the subjects of case studies are often unique individuals with specific characteristics, applying the findings to the general population can be difficult. Though case studies can offer valuable insights, their utility in explaining behavior across a broader spectrum is limited.