A researcher wants to investigate a single phenomenon to analyze issues significant to the history of an entity under study. What is this type of investigation known as?

The type of investigation described is known as a case study. A case study focuses on an in-depth analysis of a single entity, event, or phenomenon over time, allowing the researcher to explore its complexity and context. This method is particularly useful in examining issues that hold historical significance, as it facilitates a deeper understanding of the nuances that may influence the entity's development and behavior.

Case studies enable researchers to gather comprehensive data through various sources such as interviews, observations, and document analysis, leading to rich, qualitative insights. This approach is often employed in disciplines like psychology, sociology, and education to explore unique cases that may not be generalizable but offer valuable learning opportunities about specific instances.

In contrast, survey research typically involves the collection of data from a larger population to identify trends and patterns but lacks the depth and focus of a case study. Longitudinal studies track changes over time across multiple subjects, while cross-sectional studies provide a snapshot of data at a single point in time, neither of which is tailored to the in-depth exploration of a single phenomenon as effectively as case studies are.