The 100ft cloudbase I would say. For an ILS (assuming they have one) you need at least 550m and 200ft, at the planning stage (before departure). Depending on the airport and runway, these minima could be higher aswell. Depending on the airport and runway, the 100ft ceiling could be enough for a CAT II. If available a CAT III would be even better.
If the question states that you are airborne and preparing for approach, than at most airlines the limiting factor to be ALLOWED to start the approach is the RVR (if before the final approach fix).
I hope this is not to confusing for you, especially if you have no airline experience yet. I would stick to the first situation.