**Core Concept**
The question pertains to human development, specifically focusing on the **growth velocity** of infants postnatally. Growth velocity refers to the rate of growth, which can be measured in terms of increase in length, weight, or head circumference over time.

**Why the Correct Answer is Right**
The maximum growth velocity in infants occurs during the first year of life, with the most rapid growth happening in the first few months. This period is crucial for development, and factors such as nutrition and overall health play significant roles in determining growth patterns.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because, although growth does occur, it's not typically when the maximum velocity is observed.
**Option B:** Similarly, this option does not align with the period of maximum growth velocity.
**Option C:** This option also does not represent the correct timeframe for the maximum growth velocity in infants.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the first year of life, especially the first six months, is a period of rapid growth and development. Any factors that negatively impact this period can have long-lasting effects on a child's health and development.

**Correct Answer:** D. First year of life.