Answer
A REFLEX ACTION is an automatic, involuntary and rapid response of the body to a stimulus, brought about without the conscious will of the individual, and controlled by the SPINAL CORD rather than the brain.
The essential words are AUTOMATIC, INVOLUNTARY and RAPID — it happens without the person deciding to do it.
WHY THE SPINAL CORD RATHER THAN THE BRAIN: the impulse takes the short route through the spinal cord and back, which saves the time a journey to the brain would cost. That is the whole purpose — you pull your hand off a hot pan BEFORE you consciously feel the pain, which is why the brain is informed afterwards.
The pathway is the REFLEX ARC: receptor → sensory neuron → spinal cord → motor neuron → effector → response.
Adding one example secures the mark.
Example: instantly withdrawing the hand on touching a hot object; blinking when something approaches the eye; sneezing.