A ball of mass m is connected to a ball of mass M by means of a massless spring. The balls are pressed so that the spring is compressed. When released, the ball of mass m moves with an acceleration a. The magnitude acceleration of mass M will be equal to
maM
When the balls are released, force experienced by mass m is F = ma.
This is the force exerted by mass M on mass m. From Newton's third law of motion, mass m will exert the same force F on mass M in opposite direction. Therefore, the acceleration of M is a′=FM=maM and the correct choice is (c).