Which of the following property/properties of a proton can change while it moves freely in a magnetic field? (There may be more than one correct answer):

(a) mass

(b) speed

(c) velocity

(d) momentum

A proton when moves in a magnetic field, magnetic force is exerted on it. Due to this force, its velocity can change. And, since momentum is dependent on velocity, momentum can also change. Thus, options (c) and (d) are correct.