The sentence would be "Whose dirty underwear is this?".
I assume that the base (is that called deep structure sentence?) would be "This is whose dirty underwear" but I'm not sure what transformational rules are applied here.
My attempt so far is as follows:
*I should also note that I'm following "An Introduction to Language by Fromkin and others", and the notion it seems to be following is a bit simplified version of the X-bar theory, and my knowledge is pretty much limited to that.