**Core Concept**
Birth length doubling time is an important milestone in pediatric growth and development. It is a reflection of the growth velocity of the child, which is influenced by various factors including genetics, nutrition, and overall health.

**Why the Correct Answer is Right**
The birth length of a child doubles at approximately 4 months of age. This can be attributed to the rapid growth and development of the child during infancy, particularly during the first few months of life. The growth velocity is influenced by the hormonal changes, particularly the increase in growth hormone and thyroid hormone, which stimulate the growth of bones and muscles.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the birth length doubles at a younger age, specifically around 4 months, not 6 months.
* **Option B:** This option is incorrect because the birth length doubles at a younger age, specifically around 4 months, not 5 months.
* **Option C:** This option is incorrect because the birth length doubles at a younger age, specifically around 4 months, not 7 months.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the growth velocity of a child is influenced by various factors, including genetics, nutrition, and overall health. Monitoring the growth of a child is crucial to detect any potential growth disorders or developmental delays.

**Correct Answer: C. 4 months.**