## **Core Concept**
The question pertains to the growth pattern of infants, specifically focusing on the doubling of birth length. This is related to **human growth and development**, particularly **pediatric anthropometry**.

## **Why the Correct Answer is Right**
The birth length of a baby typically doubles by the age of 4 years. At birth, the average length of a baby is about 50 cm. By 4 years of age, this length approximately doubles to around 100 cm. This growth pattern is a general guideline and can vary slightly among individuals. The **doubling of birth length by 4 years** is a key milestone in pediatric growth assessment.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because the birth length does not double by 2 years of age. At 2 years, the average height is around 86-88 cm for boys and 85-87 cm for girls, which is not double the birth length.
- **Option B:** This option suggests 6 years, which is too late for the doubling of birth length. By 6 years, the average height is about 112 cm for boys and 109 cm for girls, exceeding double the birth length.
- **Option C:** This option is correct, as explained, but we'll evaluate the last option for completeness.
- **Option D:** This option suggests 10 years, which is far too late for the birth length to double. By 10 years, most children have grown significantly more than double their birth length.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that a child's **birth length doubles by 4 years** and **triples by 12 years**. This is a simple yet critical milestone in assessing growth and development.

## **Correct Answer:** .