Reflex arc refers to the pathway through which a reflex travels from the site of stimulus to sensory neuron to motor neuron to reflex muscle movement. A sensory neuron functions to carry the stimulus from the receptor site whereas a motor neuron carries signals to a muscle or organ for reflex action. In the given diagram A is the receptor, B is sensory neuron, C is a Synapse and D is an effector muscle connected to a motor neuron.