Which type of research focuses on changes over a prolonged period?

The presence of the term "changes over a prolonged period" directly points to longitudinal studies as the correct answer. Longitudinal studies are designed to observe the same subjects or phenomena over an extended timeframe, allowing researchers to track how variables change or develop over time. This method is particularly beneficial in understanding trends, developmental changes, and long-term effects, as it provides a dynamic view of the data rather than a snapshot.

In contrast, cross-sectional studies collect data at a single point in time, making them useful for assessing relationships between variables at a specific moment but not for observing changes over time. Correlational studies examine the relationship between two or more variables but do not imply a time dimension, thus not being concerned with changes over periods. Field experiments involve manipulation of variables in real-world settings but are typically focused on immediate effects rather than long-term changes.

Therefore, the correct answer highlights the unique advantage of longitudinal studies in capturing temporal changes, making it essential to understanding developments in communication research and other fields.