I would probably say "only if they have sex with someone else."
The safe choice is too pick the "if they tell someone else they have feelings," because then you can deal with it early before cheating becomes a real issue. Talk it out, and if you can't work it out, separate and move on. Or stay together and hope for good results.
Anything else on the list can be misconstrued as something simpler, though I've known people who would probably choose "if they were friends."
... granted I've never been in a genuine relationship before so its all a moot point.