Constant speed i.e. Acceleration will be zero. Kinetic energy = mv^2/r. kinetic energy is directly proportional to velocity. if velocity is constant then kinetic energy will also be constant.

Explanation:
When a body moves in a horizontal circle with a constant speed, there are certain quantities that remain constant. Let's discuss each option to determine which one remains constant.

a) Velocity:
Velocity is a vector quantity that describes the rate of change of displacement with respect to time. In circular motion, the velocity of the body is constantly changing because the direction of motion is changing. Therefore, velocity is not constant in this case.

b) Acceleration:
Acceleration is the rate at which the velocity of an object changes with time. In circular motion, the direction of velocity is constantly changing, which means there is always a change in velocity. Therefore, acceleration is not constant in this case.

c) Centripetal Force:
Centripetal force is the force that acts towards the center of the circle, keeping the body in circular motion. In this case, the centripetal force is provided by some external force (e.g., tension in a string, gravitational force, friction) and it depends on the mass of the body, speed, and radius of the circle. Therefore, centripetal force is not constant in this case.

d) Kinetic Energy:
Kinetic energy is the energy possessed by an object due to its motion. It is given by the equation: KE = (1/2)mv^2, where m is the mass of the body and v is its velocity. In circular motion, the speed (magnitude of velocity) remains constant, even though the direction of velocity changes. Since the mass of the body also remains constant, the kinetic energy of the body remains constant throughout the motion. Therefore, kinetic energy is constant in this case.

Conclusion:
In a body moving with constant speed in a horizontal circle, the only quantity that remains constant is the kinetic energy.