The correct option is A periodic but not simple harmonic
Body collides elastically with walls of room. So, there will be no loss in its energy, and it will remain colliding with walls of room, so its motion will be periodic. But here, there is no restoring force. So, the characteristics of SHM will not be satisfied.
There is no change in energy of the body, hence there is no acceleration, so its motion is not SHM.
Final answer: (b)