Which of the following is NOT a component of a reflex arc?

A reflex arc is a neural pathway that mediates a reflex action. It involves several components, including a receptor, a sensory neuron,a system (CNS), an integrating centre, and a motor neuron and effector. 

The stimulus is the external or internal environmental change that triggers the reflex. It is not considered a component of the reflex arc, but rather the initial trigger that initiates the sequence of events.

Hence, the correct option is stimulus.