Acoach wants to see how long the members of the water polo team can hold their breath. he times each member and finds that the average is 52 seconds. what type of study is this?

The type of study that this is is observational study.
He is observing his team, each one of the members, in order to see how long they can hold their breath - which is why this is an observational study. Nothing is random here, there is no hypothesis, and there is no double blind, which means that the other options are incorrect.

5th to stop bare arms