The stretch reflex is best defined as:

The stretch reflex is characterized as a reflex contraction of the muscle that occurs when muscle spindles detect a quick stretch. Muscle spindles are sensory receptors located within the belly of muscles and are sensitive to changes in muscle length and the rate of that length change. When a muscle is rapidly elongated, the muscle spindles send signals to the spinal cord, which then triggers a rapid contraction of the same muscle to resist the stretching. This protective mechanism helps maintain muscle tone and posture, preventing excessive stretching and potential injury.

In the context of this question, the other options do not accurately describe the stretch reflex. Blood pooling in the extremities pertains to circulation issues and is unrelated to the neurological response involved in the stretch reflex. PNF (Proprioceptive Neuromuscular Facilitation) stretching is a technique used to improve flexibility and does not define the physiological mechanism of the stretch reflex. Lastly, the role of the golgi tendon organs mentioned in one of the choices pertains to a different reflex pathway that primarily inhibits muscle contraction in response to excessive tension, rather than promoting a contraction as seen in the stretch reflex.