one-option rule
nounUK /wʌn ˈɒpʃən ruːl/
US /wʌn ˈɑːpʃən ruːl/
A rule that says you can only pick one thing.
The teacher used the one-option rule and told us we could only choose one book.
OTHER MEANINGS
2. A situation where only one possibility is available.
With the one-option rule, the only snack left was an apple.
3. A game rule that gives players just one move.
In this board game, the one-option rule means you always move forward one space.
4. A way of deciding that removes all other choices.
The one-option rule made the vote easy because there was only one name on the list.