Answer: The correct answer is D-3 m/s.
Step-by-step explanation: To find the velocity of the second ball, we need to use the principle of conservation of kinetic energy.
The total kinetic energy before the collision is equal to the total kinetic energy after the collision.
Initial kinetic energy = $\frac{1}{2} * 6 * v^2$
Final kinetic energy = $\frac{1}{2} * 6 * 6 + \frac{1}{2} * 6 * v^2 = 75$
Solving for v, we get v = 3 m/s.