Which of the following is a completely inelastic collision? Two balls collide, stick to each other, and move together after the
collision.

Two balls collide and move at a right angle to each other after the
collision.
A ball with velocity v collides with a ball at rest; after the collision the
first ball is at rest and the second ball has velocity v.
a ball rebounds against a wall, reversing its velocity.
Two balls collide head on, each reversing its velocity.