In my view, "had wanted" and "had been wanting" are both valid answers and mean near enough the same thing. The continuous tenses of verbs that correspond to discrete actions can describe repetition of the action, which is the case here.
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GPYThe continuous tenses of verbs that correspond to discrete actions can describe repetition of the action, which is the case here.This comment was in relation to the "marry me" sentence. Sorry, I forgot to make that clear.