Concept: This question tests your understanding of the components involved in a simple reflex arc, specifically the knee-jerk reflex, focusing on the roles of the receptor and effector.

Why (C) is correct: In the knee-jerk reflex, the stimulus (tapping the patellar tendon) stretches the quadriceps muscle. This stretch is detected by the muscle spindle, which acts as the receptor. The response involves the contraction of the quadriceps muscle, which is brought about by the motor neuron releasing neurotransmitters at the motor end plate (neuromuscular junction) on the muscle fibers. Thus, the muscle itself, activated via the motor end plate, is the effector.

Why other options are wrong:

A) Muscle spindle and motor end plate, respectively: This reverses the roles. The muscle spindle is the receptor, and the motor end plate is part of the effector mechanism.

B) Afferent neuron and efferent neuron, respectively: These are the neurons involved in transmitting the signal, not the receptor and effector themselves.

D) Sensory neuron and motor neuron, respectively: Similar to option B, these are the neural components, not the receptor and effector organs.

Common Mistake: Students often confuse the neuron types (sensory/motor) with the actual receptor and effector structures.

NEET Tip / Mnemonic: Remember the flow: Stimulus $\to$ Receptor $\to$ Afferent neuron $\to$ CNS $\to$ Efferent neuron $\to$ Effector $\to$ Response. The receptor detects, the effector acts.

Correct Answer: (C)