A

In an elastic collision, both momentum and kinetic energy are conserved. For a ball colliding with the floor, we can analyze the system before and after the collision.
Step 1: Define the system. Let the ball have a mass m and an initial velocity u (vertically downward) just before it hits the floor. Its momentum before the collision is given by:
$$p_{initial} = m imes u$$
Step 2: Analyze the collision. After colliding elastically with the floor, the ball will rebound with the same speed but in the opposite direction, at velocity v = -u. The momentum after the collision is:
$$p_{final} = m imes (-u) = -m imes u$$
Step 3: Apply conservation of momentum. In the absence of external forces, the total momentum before and after the collision for the ball must be equal when considered with the Earth's reaction. Although the ball changes its momentum, the floor (Earth) does not change speed but absorbs the momentum change.
Hence, momentum is conserved in the overall system, which includes both the ball and the Earth as the other body involved in the collision.
Therefore, the statement that the momentum of a ball colliding elastically with the floor is conserved is indeed correct.