The sentence Ils sont partis ___ la maison means "They left ___ the house." This question tests the correct preposition to use with the verb partir (to leave).
The correct answer is de. The verb partir takes the preposition de to indicate the place one is leaving from. Partir de means "to leave from" or "to depart from." The sentence Ils sont partis de la maison means "They left (from) the house."
The other prepositions change the meaning or are not standard with partir in this context:
vers (toward) would indicate direction, not departure: vers la maison means "toward the house"
a (to/at) is not the standard preposition for indicating departure from a place
pour (for) could indicate a destination (partir pour Paris = to leave for Paris), but partir pour la maison is not the natural construction here
The verb-preposition combination partir de (to leave from a place) is essential to know, alongside venir de (to come from) and sortir de (to go out of).
