In order to produce a reflex, a stimulus must be present.
So 1 is the stimulus.
Receptors sense the stimulus.
So 2 must receptor.
The receptors send electrical impulses to CNS via sensory neurones.
3 must be CNS.
CNS then relays these electrical impulses to the muscles via motor neurones.
4 must be muscle.
Muscle then contracts to produce a movement (response).
5 must be response.
Hence B is correct.