Label the parts $(a)$,$(b)$,$(c)$ and $(d)$ and show the direction of flow of electrical signals in the figure.

(N/A) The given figure represents a reflex arc,which is the pathway taken by nerve impulses in a reflex action.
$(a)$ Sensory neuron: It carries the electrical signal from the receptor (skin) to the central nervous system.
$(b)$ Spinal cord: It acts as the processing center for the reflex action.
$(c)$ Motor neuron: It carries the response signal from the spinal cord to the effector.
$(d)$ Effector: It is the muscle in the arm that contracts to pull the hand away from the hot object.
The direction of the flow of electrical signals is from the receptor (hand) $\rightarrow$ sensory neuron $(a)$ $\rightarrow$ spinal cord $(b)$ $\rightarrow$ motor neuron $(c)$ $\rightarrow$ effector $(d)$ (muscle).