Option 1: Speed - The speed of a proton, which is the rate at which it moves, is not influenced by the presence of a magnetic field. The magnetic field only affects the proton`s direction, not its speed. Therefore, the speed of a proton will remain constant even when it moves freely in a magnetic field.

Option 2: Charge - The charge of a proton, which is positive, does not change while it moves in a magnetic field. The magnetic field may exert a force on the proton due to its charge, but the charge itself remains constant.

Option 3: Mass - The mass of a proton, which is a fundamental property, does not change while it moves in a magnetic field. The magnetic field affects the proton`s motion, but it does not alter its mass.

Option 4 (Correct Answer): Velocity - The velocity of a proton, which is its speed in a given direction, can change while it moves freely in a magnetic field. This is because the magnetic field exerts a force on the proton, causing it to change its direction of motion. As a result, the velocity, which takes into account both speed and direction, can change when the proton moves in a magnetic field.

It is important to note that the mass, charge