If a force of constant magnitude acts in direction perpendicular to the motion of a particle, then the particle follows circular motion. ... And in circular motion speed is uniform but the velocity is not uniform because its direction will change repeatedly. As the velocity is not uniform momentum will also change.

Explanation:

When a force of constant magnitude acts in a direction perpendicular to the motion of a particle, several properties of the particle are affected. Let's analyze each option in detail:

(a) Speed is uniform:
When a force acts perpendicular to the motion of a particle, it does not change the speed of the particle. The force only changes the direction of the particle's motion. Since the magnitude of the force remains constant, it does not cause any change in the speed of the particle. Therefore, the speed of the particle remains constant.

(b) Velocity is uniform:
Velocity is a vector quantity that includes both magnitude and direction. When a force acts perpendicular to the motion of a particle, it changes the direction of the particle's motion but does not affect its speed. As a result, the velocity of the particle changes because the direction changes, but the magnitude of the velocity remains constant. Therefore, the velocity of the particle is uniform.

(c) Momentum is uniform:
Momentum is defined as the product of an object's mass and its velocity. When a force acts perpendicular to the motion of a particle, it does not change the magnitude of the particle's velocity. Since the magnitude of the velocity remains constant, the momentum of the particle remains constant as well. Therefore, the momentum of the particle is uniform.

(d) All of these:
From the explanations above, it is clear that all of the options (a), (b), and (c) are correct. The speed remains uniform because the force does not change the magnitude of the particle's velocity. The velocity remains uniform because the magnitude of the velocity remains constant. The momentum remains uniform because the magnitude of the velocity remains constant, and momentum is the product of mass and velocity.

In conclusion, when a force of constant magnitude acts in a direction perpendicular to the motion of a particle, the speed, velocity, and momentum of the particle all remain uniform. The force only changes the direction of the particle's motion, while the magnitude of the velocity and momentum remain constant.