Final answer:
A proton moving west in a uniform magnetic field directed upward will follow a circular path that lies in a horizontal plane due to the perpendicular magnetic force it experiences as per the right-hand rule.
Step-by-step explanation:
When a proton is moving west in a uniform magnetic field that is directed vertically upward, the proton will experience a magnetic force due to its motion through the magnetic field. This magnetic force will be perpendicular to both the direction of the proton's velocity and the magnetic field (right-hand rule). As the force is always perpendicular to the velocity, the proton will begin to move in a circular path with a radius determined by the balance of this magnetic force and the proton's momentum.
For a proton moving westward in a uniform magnetic field directed upwards, the magnetic force will act in a direction that is towards the north or south depending on the orientation of the magnetic field relative to the proton's velocity. This circular motion will lie in a horizontal plane. The shape of the trajectory is a circle, and the orientation of this circle is in the horizontal plane perpendicular to the magnetic field.