When a charged particle enters a magnetic field in a direction perpendicular to the field, several properties of the particle are affected. However, the energy of the particle does not change in this scenario.

Velocity of the Particle:
When a charged particle enters a magnetic field perpendicular to its direction of motion, it experiences a force known as the Lorentz force. This force acts perpendicular to both the magnetic field and the velocity of the particle, causing the particle to move in a circular path. The velocity of the particle changes because it now has a component in the direction of the magnetic field and a component perpendicular to it. This change in velocity is caused by the force exerted by the magnetic field.

Mass of the Particle:
The mass of a particle is an intrinsic property and does not change when it enters a magnetic field. The mass of a charged particle is determined by its fundamental properties and is not influenced by the presence of a magnetic field.

Energy of the Particle:
The energy of a particle is determined by its velocity and mass. Since the velocity of the particle changes when it enters a magnetic field, the energy of the particle also changes. The circular motion induced by the magnetic field results in a change in the kinetic energy of the particle. Therefore, the energy of the particle is not conserved in this scenario.

Momentum of the Particle:
The momentum of a particle is defined as the product of its mass and velocity. When a charged particle enters a magnetic field, its velocity changes due to the Lorentz force. As a result, the momentum of the particle also changes. The direction and magnitude of the momentum are altered by the magnetic field.

In summary, when a charged particle enters a magnetic field in a direction perpendicular to the field, the velocity, momentum, and energy of the particle are affected. The mass of the particle, however, remains unchanged. Therefore, the correct answer is option 'C' - the energy of the particle does not change.