Introduction:
When a satellite in a circular orbit around the Earth enters the atmospheric region, it encounters small air resistance to its motion. This resistance causes changes in the satellite's motion and energy. Let's explore the effects of air resistance on the satellite's kinetic energy, angular momentum, and period of revolution.

Explanation:

1. Kinetic Energy:
When a satellite enters the atmospheric region, it experiences air resistance, which acts opposite to its direction of motion. This air resistance does negative work on the satellite, which means it removes some amount of kinetic energy from the satellite.

Since kinetic energy is given by the equation KE = (1/2)mv², where m is the mass of the satellite and v is its velocity, the decrease in kinetic energy can be attributed to a decrease in velocity. As the satellite loses kinetic energy, its velocity decreases, and as a result, the kinetic energy decreases. Therefore, option 'A' is correct.

2. Angular Momentum:
Angular momentum is a conserved quantity for an object moving in a central force field like gravity. The angular momentum of a satellite in a circular orbit around the Earth remains constant as long as no external torque acts on it.

When the satellite encounters air resistance, it experiences a force opposite to its motion. This force does exert a torque on the satellite, causing a change in its angular momentum. However, since the torque is small, the change in angular momentum is negligible, and the angular momentum about the Earth remains nearly constant. Therefore, option 'C' is incorrect.

3. Period of Revolution:
The period of revolution of a satellite is the time it takes to complete one orbit around the Earth. It is determined by the satellite's orbital radius and the gravitational force acting on it.

When the satellite encounters air resistance, it experiences a decelerating force. This force reduces the satellite's speed, which affects its orbital radius. As the satellite's speed decreases, its orbital radius decreases as well. Consequently, the period of revolution decreases because the satellite takes less time to complete each orbit. Therefore, option 'D' is incorrect.

Conclusion:
When a satellite in a circular orbit around the Earth enters the atmospheric region, it encounters air resistance that causes its kinetic energy to decrease. However, its angular momentum remains nearly constant, and the period of revolution decreases due to a decrease in the orbital radius. Therefore, the correct answer is option 'A'.

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