The correct option is A Skin, muscles
The event of hand touching a hot object and taken back immediately is an example of reflex action. Here, heat acts as a stimulus, skin of the hand is the receptor organ and the muscles responsible for the movement of the hand are the effector organs. The sensory neurons have nerve endings in the skin which carry the impulse, created due to heat, to the spinal cord. The relay neuron transmits it to the effector/motor neuron which makes the muscles move and hand is taken back.