If someone were to casually ask, "What did he want?", as if they were asking the waiter, the response would be "Tea with milk and sugar" but that's not quite the case. The correct question should be, "What did the customer order?". "Want" suggests what he desired which could cause some students to select 3. In the case of your test, the question using "want" could both be used as "what did he want" and "what did he order?" Therefore, it would be best to specify what the question's true query is, such as, "What was the customer's preferred beverage?" or "What did the customer order?" In this way, the ambiguity is avoided.