My brain said "y" and then I caught myself. Well done!
(I suppose my context was primed both by your brain-teaser, and also the fact that we've been talking about these sorts of things. If you'd said this to me out of the blue, I probably would have spelled out all of "yolk" and thought it was correct.)
A funny riddle is a j-o-k-e that sounds like “joke”.
You sit in the tub for an s-o-a-k that sounds like “soak”.
So how do you spell the white of an egg?
// All of these prove humans are subject to "context priming".