The magnetic force on a moving proton acts towards north in a horizontal plane. If the proton is moving vertically upward then what will be the direction of magnetic field?

The direction of moving proton is the direction of current. Hence according to question,

The current is in the upward direction with respect to the horizontal plane

Magnetic force is acting in the direction of north on the horizontal plane.

Hence, from Flemings left-hand rule we can determine that the magnetic field is in the direction of East on the horizontal plane.