Answer:
Any one of these three works:
plane MOU
plane MNU
plane NOU
Explanation:
A plane can be named by a single letter, such as L in this problem, or by any three non-collinear points that lie on the plane. Non-collinear points are points that do not all lie in a single line.
Points M, N, O, and U lie on plane L, so you can choose any 3 of the 4 points to name the plane with, but make sure all 3 points are non-collinear.
To name plane L with points, you cannot use points MNO together since they are collinear, but you can name it using point U plus any two of the points M, N, and O.
plane L can be named
plane MOU
plane MNU
plane NOU
Do not name it plane MNO