In which sentence is a homophone used correctly?
a. if you ask me, there's no hobby like fishing.
b. karen's new dog had flees.
c. the old miner was lost in the dessert.
d. how many ours do we have to wait?

I think the answer is A. If you ask me, there's no hobby like fishing. because  
In c, it should be desert because dessert is like ice-cream and cakes. 
In d, it should be hours because ours is a possessive pronoun. Hours is time. 
In b, it should be fleas because flees is a verb where people run away. Fleas is the parasite dogs get. 
In A, it is perfectly correct!