When a charged particle, such as a proton, enters a magnetic field at a right angle, it experiences a force due to the interaction between its charge and the magnetic field. The direction of the force can be determined using the right-hand rule.

To apply the right-hand rule, extend your right hand with your thumb, index finger, and middle finger perpendicular to each other. If you align your thumb to represent the direction of the velocity of the proton, and your index finger to represent the direction of the magnetic field, then your middle finger will indicate the direction of the force.

In this question, the proton enters the magnetic field at a right angle, so its velocity is perpendicular to the field. As a result, the force acting on the proton will be in the direction indicated by the middle finger, which is into the page. Therefore, option 4, "into the page," is the correct answer.

The magnetic force experienced by a charged particle can be calculated using the formula F = qvBsin[REPLACEMENT], where F is the force, q is the charge, v is the velocity, B is the magnetic field strength, and [REPLACEMENT] is the angle between the velocity and the magnetic field. In this case, since the angle is 90 degrees, sin[REPLACEMENT]