Assuming this is a direction to the class, I'd say: "You must use the verb with this preposition." "Mandatory" is possible. "Use of this word is mandatory." The others sound wrong when used here. "Are obliged to" is not as direct and is old-fashioned. "The use of this word is imperative" sounds like very bad things will happen if you don't. "Compulsory" generally means it is required by some authority. "Your attendance in class is compulsory." The simple, clear word, "must" should be used here.