Hi manimani,
Congratulations on starting your GMAT prep!
The "conclusion" of the argument is the author's main point. You can identify it by keywords such as "therefore" or "thus," or by identifying a major opinion advanced by the author. In this case, the first sentence, that Hip-Hop is underrated, is the author's conclusion.
"Evidence", also known as "premises", are any and all points the author offers to support his or her conclusion. These are often data, facts, and numbers, but not necessarily. In this case, the last sentence justifies his conclusion that Hip-Hop is underrated: the reason he thinks so is that people judge hip-hop based on it's poor-quality parts, but you can't just a whole by just certain parts.
For that reason, (C) is correct.
There are plenty of ways to get more practice. The
opening thread in this forum links to several free guides. Kaplan GMAT Premier has a comprehensive method and breakdown of question types. And, Kaplan's
blog has lots of CR tips to help you out.
Good luck on your prep, and keep posting good questions!
Regards,