You've noticed that as the temperature drops outside, you see more students wearing sweaters and heavy coats. your observation is most similar to

The answer is a. correlational research.

In this scenario, you only do observation of one event and trying to find a link between them. This method is good for finding correlation, but not for causation.

Because you only observe the subject without intervention, it shouldn't be experimental research. What you observe is only one condition, so there is no control subject that get different condition.