Which of the following property of a proton can change while it moves freely in a magnetic field?

When a proton moves freely in a magnetic field, the property that can change is its velocity. The correct answers are:

(c) velocity
(d) momentum

The mass of a proton remains constant, and the speed can also remain constant if the magnitude of the velocity remains the same. However, the direction of the velocity can change, leading to a change in momentum, which is a vector quantity dependent on both magnitude and direction. Therefore, both velocity and momentum can change when a proton moves freely in a magnetic field.