What type of research design is least effective for showing changes over time?

The correct choice is a cross-sectional study. This type of research design collects data at a single point in time, which means it provides a snapshot of the population being studied but does not observe how variables change over time. Because it lacks a temporal dimension, it does not allow researchers to establish any trends or causal relationships regarding changes throughout a specified period.

In contrast, longitudinal cohort studies actively track the same subjects over an extended time frame, making them effective in identifying changes and developments within those subjects. Experimental studies typically manipulate an independent variable over time to observe effects, facilitating an understanding of change. Case-control studies compare subjects with a condition to those without, but they are retrospective and do not inherently show change over time in the same way longitudinal studies do.