Final answer:
The stretch reflex is a monosynaptic, rapid response that causes a stretched muscle to contract (Option A).
Step-by-step explanation:
The reflex you are describing is known as a stretch reflex. This is a simple, monosynaptic reflex arc that involves a sensory neuron and a motor neuron without any interneuronal connections. It occurs when a muscle length is increased unexpectedly, which then prompts the muscle spindles to send a signal through their sensory neuron back to the spinal cord where it synapses directly onto a motor neuron.
This motor neuron provokes the muscle to immediately contract to counteract the stretching. A collateral branch of the afferent fiber also communicates with interneurons to inhibit the motor neurons of antagonistic muscles. A classic example of the stretch reflex is the knee-j*rk response, which helps maintain posture and balance by causing muscles to contract when they are stretched.
Thus, the correct option is A.