Answer: A la frutería.
Explanation: The sentence that we must complete is:
Voy ______ para comprar una sandía, unos melones y unas manzanas.
And this means something like:
I will go to_______ to buy an
watermelon, some melons, and some apples.
The right option is "A la frutería", that means "At the fruit store", which is the place where Héctor will find the things he wants to buy.