**Core Concept:**
In the postnatal period, the brain undergoes significant growth and development. The Central Nervous System (CNS) includes the brain and spinal cord, which undergo continuous growth and maturation during early childhood. The CNS growth and development are essential for cognitive, motor, and sensory functions.

**Why the Correct Answer is Right:**
The greatest growth in the CNS during the postnatal period occurs in the first two years of life. This period is known as the critical period for brain development, where the brain undergoes myelination (formation of myelin sheaths around axons), synaptogenesis (formation of synapses), and neural pruning (removal of unnecessary connections).

In the correct answer (D), we focus on a specific aspect of CNS growth: myelination. Myelination is the process where the myelin sheath is formed around axons, improving the conduction velocity of nerve impulses, which is essential for normal brain function and development.

**Why Each Wrong Option is Incorrect:**
A) The first two years of life: While this is a crucial period for CNS development, it is not the only time of significant growth.
B) CNS growth occurs throughout life: Although brain volume decreases with age, the process of myelination continues, but at a slower rate after the first two years.
C) Growth plate in long bones: This is a different anatomical structure involved in bone growth, not related to CNS growth.

**Clinical Pearl:**
Postnatal brain growth is essential for optimal cognitive and motor development. Ensuring proper nutrition, particularly with omega-3 fatty acids and vitamin D, can positively impact brain development in infants and children. Monitoring and addressing any potential risks, such as exposure to environmental toxins or infectious diseases, is crucial for optimal CNS development.

**Correct Answer:** D) The first two years of life.