The question asks for the opposite (le contraire) of facile (easy). The standard antonym of facile in French is difficile (difficult). Since difficile is not among the options, the closest synonym is dur.
The adjective dur has two main meanings in French: "hard" in the physical sense (a hard surface) and "hard/difficult" in the abstract sense (a hard task). In its second meaning, dur functions as a synonym of difficile and therefore serves as an antonym of facile. For example: "Cet exercice est dur" = "This exercise is hard/difficult."
The other options are not antonyms of facile:
petit means "small" - the opposite of grand.
lourd means "heavy" - the opposite of leger (light).
mechant means "mean/nasty" - the opposite of gentil (kind).
