And if you could to fulfill only one desire, what is it?
p.s. Tell me about mistake in sentences, please, if it have. :)
Ask him about the secret of life, the universe and everything. If he says 42 then he will be of no use to me :)
Desire is not suitable here in place of wish here.
(Imagine you found a lamp with a genie inside, and you could fulfill any three desires - or - and you were granted three wishes, what would they be?)
if you could fulfill/satisfy only one desire, what would it/that be?
<em>when you're dealing with a genie in a lamp situation, it's usually "three wishes", a bit simpler that way</em>