Answer:
Magnetic field,

Step-by-step explanation:
It is given that,
Speed of proton,

Mass of the proton,

Charge on proton,

We need to find the magnetic field strength required to just balance the weight of the proton and keep it moving horizontally.
The Lorentz force is given by :
.............(1)
The weight of proton,
..............(2)
From equation (1) and (2), we get :




Hence, this is the required solution.