Which of the following proton characteristics may change when it flows freely in a magnetic field?

 Electric charge is the property of matter that is responsible for electrostatic force.

A positively charged particle is a proton.

The magnetic field of a magnet is the region around a magnet in which the magnetic force may be felt by any other magnet or magnetic substance. Magnetic force on any moving charged particle $\left(\mathrm{F}\right)=q\overrightarrow{V}\times \overrightarrow{B}$

Where q is a charge, $\overrightarrow{V}$ its velocity vector, and $\overrightarrow{B}$ is magnetic field vector

$\overrightarrow{V}\times \overrightarrow{B}$ is the cross product of velocity vector and magnetic field vector

Magnetic force on any moving charged particle $\left(\mathrm{F}\right)=q\overrightarrow{V}\times \overrightarrow{B}$

Because magnetic force is directed in the direction of the cross product of velocity and magnetic field, which is perpendicular to the charged particle's motion.

Because the magnetic force on every moving charged particle is perpendicular to its velocity. As a result, the magnetic force has no effect on the magnitude of velocity. It just changes the velocity's direction.

Because the magnitude of the velocity equals speed. In a magnetic field, the speed of a traveling charge particle remains constant. As a result, option 1 is inaccurate.

The mass of the thing is another constant feature.

The fee remains unchanged. Option B is thus incorrect.

The velocity varies when the direction of motion changes. As a result, option D is right.