A) CLEAN! Seriously, it is very, very offensive to me and other players if you turn up with underpants that are shall we say not in the best condition in the world..ok, I will be blunt have skid marks down the back!!! You can pick a pack of 3 pairs up from any shop that are not expensive so there is no need for you to be unhygienic.
I do have an underwear fetish (there's a surprise) I love to see the one's that fit snuggly to the bottom, most have lycra or elastine mix in that allows this, they frame a bottom so well and of course they are brilliant for spanking over <grins>