Which of the following describes an object in dynamic equilibrium? A. A leopard slows down from 10 m/s to 5 m/s.
B. A cheetah runs 15 m/s to the south
C. A dog runs in a circle at a speed of 8 m/s.
D. A cat sits on a bed without moving

The second option: A cheetah runs at to the south (assuming that the speed of this animal is constant.)

Explanation:

An object is in dynamic equilibrium if and only if the velocity of this object is constant and non-zero.

Since the velocity of an object is a vector, velocity is constant if and only both its direction and its magnitude (that is, the speed of the object) are constant.

In the first choice, the magnitude of the velocity of this leopard is changing. Hence, the velocity of this animal is not constant, and this animal is not in an equilibrium.

In the second choice, the velocity of the animal is indeed constant.

In the third choice, the direction of the velocity of the animal kept changing. Hence, the velocity of this animal is not constant, and this animal isn't in an equilibrium.

In the fourth choice, the velocity of the animal is zero. Hence, this animal is in a static equilibrium rather than a dynamic one.