Answer:
The diameter of the circular path decrease
Step-by-step explanation:
The magnitude of the magnetic force exerted on the proton is given by:

According to Newton's second law, this force is equal to the centripetal force. Therefore, we have:

Recall that
. Replacing and solving for B:

Thus, the field stregth is inversely proportional to the radius of the path and since the radius is directly proportional to the diameter, if the field strength increases, the diameter decreases.