The drop-down hints are just that: hints. You must use your knowledge of the French word and its use to correctly construct an articulate sentence in English. The use of the imperfect tense is a little odd here so the best thing is not to try to make it more complicated: "You did not know that cook."
You're right. Verbs in imparfait can be translated into English as "used to verb" and "was verb-ing". However, English seldom if ever allows "was knowing." But "used to read" and "was reading" are both acceptable in English and can actually mean two different thanks. So you must translate to English in a way that makes sense to your ears yet keeps the same meaning implied by the French.