Before the experiment, the total momentum of the system is 2.5 kg m/s to the right and the kinetic energy is 5J. After the experiment, the total momentum of the system is 2.5 kg m/s to the right and the kinetic energy is 4J.
a)This describes an elastic collision (and it could NOT be inelastic).
b)This describes an inelastic collision (and it could NOT be elastic).
c)This is NEITHER an elastic collision nor an inelastic collision
d)This describes a collision that is EITHER elastic or inelastic, but more information is required to determine which.

B

Explanation:

Newton's third law of motion states that to every action there is equal an opposite reaction. Momentum is always conserved provided there is no net force on the body. Considering the experiment: the momentum is conserved as expected but the  kinetic energy is not conserved meaning that the collision is not elastic; some energy is converted into internal energy. When the collision is elastic the total kinetic energy before will equal total kinetic energy after and when the body stick together, they move with a common velocity and that described a perfectly inelastic collision.  5J ≠ 4J