Paper

He looks like the type of guys to punch a lot of people. Probably likes keep his fist closed right? So if he's gonna throw rock it'd be best to throw paper.

You throw Paper

Your hunch was right and he threw Rock.

Although he doesn't look all that worried about it. Maybe he's wearing a cup?

"Take your best shot" he says with a smug grin while he spreads his legs and puts his hands behind his head.

He's not wearing a cup at all! He just thinks that you can't do any real damage

"Oh don't you worry. When I'm done you'll have one less nut"