Answer:
Receptor--->sensory neuron-->interneuron ----> motor neuron---> effectors.
Step-by-step explanation:
A simple reflex arc consists of sensory neuron, interneuron of center and motor neuron. Receptors serve to sense the respective stimuli and the sensory neurons from receptors transmit the sensory information from sensory receptors to spinal cord. Interneurons of spinal cord integrates the incoming information and signal is relayed to motor neurons. The motor neuron transmits the signal from spinal cord to the effector organs which are mostly muscles and glands.
Hence the correct order of activated neurons in reflex arc is:
Receptor--->sensory neuron-->interneuron ----> motor neuron---> effectors