When a proton enters in any magnetic field then the magnetic force of that magnetic field perpendicularly works upon that proton which entered in magnetic field.

As a result,the the direction of that proton's velocity changes.

And we all know that velocity is a vector quantity and it has both direction and magnitude.

So if we change the direction then the allover value of the velocity will also change.

And the momentum is another vector unit which is dependent on the value of the velocity ,if the velocity's value changes then the change of the momentum's value is obvious.

Introduction:
When a proton moves freely in a magnetic field, it experiences a force called the magnetic force. This force can affect certain properties of the proton. Among the given options, the property that can change while the proton moves freely in a magnetic field is its velocity.

Explanation:
1. Magnetic Force:
When a proton moves freely in a magnetic field, it experiences a magnetic force. The magnetic force acting on a moving charged particle is given by the equation F = qvBsinθ, where F is the magnetic force, q is the charge of the particle, v is its velocity, B is the magnetic field strength, and θ is the angle between the velocity vector and the magnetic field vector.

2. Velocity:
Velocity is a vector quantity that describes the speed and direction of an object. It is defined as the rate of change of displacement with respect to time. The velocity of a proton moving freely in a magnetic field can change because the magnetic force acts perpendicular to the velocity vector, causing the proton to change direction.

3. Mass:
Mass is a scalar quantity that represents the amount of matter in an object. It does not change when a proton moves freely in a magnetic field. The magnetic force does not affect the mass of the proton.

4. Speed:
Speed is a scalar quantity that represents the rate at which an object covers distance. It does not change when a proton moves freely in a magnetic field. The magnetic force does not affect the speed of the proton.

5. Momentum:
Momentum is a vector quantity that represents the quantity of motion of an object. It is defined as the product of mass and velocity. The momentum of a proton can change while it moves freely in a magnetic field due to the magnetic force acting on it. The magnetic force can cause a change in the direction of the proton's velocity, resulting in a change in its momentum.

Conclusion:
Among the given options, the property of a proton that can change while it moves freely in a magnetic field is its velocity. The magnetic force acting on the proton can cause a change in its direction, resulting in a change in velocity. The mass, speed, and momentum of the proton do not change when it moves freely in a magnetic field.