**Core Concept**
The stretch reflex is a monosynaptic reflex that involves the contraction of a muscle in response to its own stretching. This reflex is mediated by the muscle spindle, which senses the length and velocity of muscle stretch.

**Why the Correct Answer is Right**
The unit of the stretch reflex is the motor neuron and the muscle spindle. When a muscle is stretched, the muscle spindle is activated, releasing the neurotransmitter glutamate, which binds to the alpha motor neuron, causing it to fire and contract the muscle. This is a monosynaptic reflex, meaning there is only one synapse between the sensory neuron and the motor neuron. The correct answer is the **Ia afferent neuron**, which is the sensory neuron that senses the stretch and activates the motor neuron.

**Why Each Wrong Option is Incorrect**
**Option A:** The **alpha motor neuron** is the motor neuron that contracts the muscle, but it is not the unit of the stretch reflex.
**Option B:** The **muscle spindle** is the sensory receptor that senses the stretch, but it is not the unit of the stretch reflex.
**Option C:** The **Golgi tendon organ** is a sensory receptor that senses muscle tension, but it is not involved in the stretch reflex.

**Clinical Pearl / High-Yield Fact**
The stretch reflex is an important component of muscle tone and is often tested in neurological exams, such as the Babinski reflex.

**Correct Answer:** C. Ia afferent neuron