## **Core Concept**
The question pertains to the growth pattern of a human, specifically focusing on the relationship between birth length and the age at which it doubles. This involves understanding human growth and development, particularly the changes in height from infancy through childhood.

## **Why the Correct Answer is Right**
The average birth length of a human is approximately 50 cm. It is a well-established fact in pediatrics that a child's birth length doubles by the age of 4 years. By 4 years, the average height of a child is around 100 cm, which is double the average birth length. This growth pattern is crucial for monitoring the development and health of children.

## **Why Each Wrong Option is Incorrect**
- **Option A:** 2 years - At 2 years, the average height of a child is roughly 85-90 cm, which is not double the birth length.
- **Option B:** 6 years - By 6 years, the average height of a child is about 110-120 cm. While this is more than double the birth length, it is not the age at which the birth length doubles.
- **Option D:** 8 years - At 8 years, children's average heights range approximately from 125 to 130 cm, which is more than double the birth length and beyond the specific milestone of doubling.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that a child's birth length doubles by the age of 4 and triples by the age of 12. This growth pattern is an essential indicator of healthy development, and deviations from these expected growth curves can signal potential health issues.

## **Correct Answer:** C. 4 years.