Which of the following proton characteristics can change when it freely flows through a magnetic field? (There may be more than one answer.)  

When a proton enters a magnetic field, it starts moving in a circular path, and each moving charged particle in a magnetic field experiences a force. This force affects the velocity and momentum of a proton. The force can change the proton’s direction (velocity) but not its speed(magnitude). As a result, the momentum and velocity of the proton change. 

Hence, the correct options are (c) Velocity and (d) Momentum.