The conclusion: It is not true that meditation causes people to perform better on the GMAT.

The evidence: Students who meditate are more likely than other students to have adequate time to study.

Assumption: Having adequate study time is the actual cause of superior scores. This is shown by correct Choice C. If meditation was the real reason, then students who meditate would do better than those who don't, even though both groups have adequate study time. That's not what happened according to the argument. When adequate study time was the same, the meditators did no better. This strengthens the conclusion that meditation does not cause people to perform better on the test.

You've only included two wrong choices. Choice A does say that meditators are more likely to give up other activities to increase study time, but does NOT say that meditation was the cause of the higher scores.

Choice B weakens the conclusion. It suggests that meditation DOES improve scores.

I'm available if you'd like any follow up.