studies

It depends on context. If the studies you are referring to are something like professional experiments or something of that nature, than you refer to them as a study. So, say a group of people did a series of studies on the amount of bacteria in a lake. Than that would be referred to as studies, which is what I used in the sentence. If he was studying for a class or something, it would be like this: He did a lot of studying, which is evident in this report. So, it really depends on what you're trying to get across.

don't study :P

He has done a lot of studies which can be found in his report.