Case of Accelerated Motion:
Acceleration is defined as the rate of change of velocity with respect to time. It occurs when an object's velocity changes, either in magnitude or direction, over a certain period of time. In accelerated motion, the velocity of the object is not constant, but rather it is changing.

The case of accelerated motion can be identified by observing the following scenarios:

Motion of the Athletes in a Straight Line:
- When athletes participate in a race, they start from rest and gradually increase their speed.
- As they run, their velocity continuously changes, and they cover more distance in less time.
- This change in velocity indicates that the athletes are undergoing accelerated motion.

Body Moving with Constant Velocity:
- In contrast to accelerated motion, constant velocity means that both the speed and direction of an object remain unchanged.
- When an object moves with constant velocity, there is no change in its speed or direction, and therefore, there is no acceleration.
- Thus, a body moving with constant velocity does not fall under the case of accelerated motion.

Uniform Circular Motion:
- Uniform circular motion refers to the motion of an object traveling in a circular path at a constant speed.
- Even though the speed remains constant, the direction of the object is continuously changing.
- As the object moves in a circular path, its velocity vector is constantly changing direction, indicating a change in velocity.
- Therefore, uniform circular motion is an example of accelerated motion.

Conclusion:
Based on the given options, only options 'a' and 'd' involve a change in velocity, indicating accelerated motion. Option 'b' states constant velocity, which does not fall under accelerated motion. Hence, option 'c' (both a and d) is the correct answer as it includes both the cases of accelerated motion.