A) When a ball hits a wall (stationary) it returns back with same velocity, if collision is elastic.
B) A ball hits a bus moving towards it. If the collision is perfect inelastic, they move together.
C) Inelastic collision arises due to loss in K.E during collision

In a collision law of conservation of momentum and kinetic energy are valid such collision is called elastic collision. 

As the velocity of approach and velocity of separation is equal in the case of ball hitting the wall it is elastic collision.

The collision between ball and bus is perfectly inelastic because in perfect inelastic collision bodies move together.

The Kinetic energy is lost in an inelastic collision.