I think the intent of the question is, "If the RVR is reported as less than 1800, what type of approach are you legal to fly?" In which case the correct answer is CAT 2 or 3. With an 1800 RVR you would not be legal to fly any non prec, therefore your situation of having the rwy in sight after the IAF is not going to happen. In fact, with an 1800 RVR you won't even have the threshold in sight at DH on a CAT I.