**Core Concept**
The cyclical flexion and extension motions of a leg during walking are a result of the coordinated contraction and relaxation of muscles in the lower limb, which is controlled by the central nervous system. This process involves the integration of sensory information from proprioceptors and the execution of motor responses to maintain posture and movement.

**Why the Correct Answer is Right**
The correct answer is the spinal cord because the walking cycle is primarily controlled by the spinal cord's ability to integrate sensory information and generate coordinated motor responses. This is achieved through the activity of the spinal cord's central pattern generators (CPGs), which are neural networks that produce rhythmic patterns of activity to control movements such as walking. The spinal cord's CPGs receive inputs from sensory receptors in the muscles, tendons, and joints, and use this information to adjust the timing and force of muscle contractions to maintain a stable and efficient gait.

**Why Each Wrong Option is Incorrect**
**Option A:** The cerebral cortex is involved in the planning and initiation of voluntary movements, but it is not directly responsible for the cyclical flexion and extension motions of walking.
**Option B:** The cerebellum is involved in the coordination and regulation of movement, but it is not the primary site of control for walking.
**Option C:** The globus pallidus is a structure within the basal ganglia that is involved in the regulation of movement, but it is not directly involved in the control of walking.

**Clinical Pearl / High-Yield Fact**
The spinal cord's ability to control movements such as walking is an example of a spinal automatism, which is a self-sustaining pattern of neural activity that can continue even in the absence of input from the brain. This is why individuals with spinal cord injuries can often continue to walk or perform other movements even after the injury.

**✓ Correct Answer: D. Spinal cord**