The sentence Ma maison se trouve _ du port ("My house is located _ the port") needs a prepositional phrase that logically describes a spatial relationship between a house and a port.
The correct answer is à côté, meaning "next to" or "beside." The complete sentence reads: Ma maison se trouve à côté du port ("My house is located next to the port"). The expression à côté de is a standard locative prepositional phrase in French, and here du is the contraction of de + le (the port being masculine).
The other options do not fit naturally:
sous means "under" - a house is not typically located under a port.
hors means "outside of" - but hors du port would describe something outside the port area, which is not a natural way to locate a house.
à moins means "unless" (a conditional conjunction), not a preposition of place. It cannot describe spatial location.
