**Core Concept**
The question tests the understanding of human development and growth patterns, specifically focusing on the comparison of organ or structure sizes at birth to their adult sizes. This concept involves **anatomical development** and **growth physiology**.

**Why the Correct Answer is Right**
Since the correct answer is not provided, I will assume a common answer for this type of question: the **brain**. The brain at birth is relatively close to its adult size, especially when compared to other organs like the liver or kidneys, which grow significantly after birth. This is because the brain's growth is largely prenatal.

**Why Each Wrong Option is Incorrect**
**Option A:** Assuming this could be the liver, it's incorrect because the liver, although functional at birth, continues to grow significantly in size after birth.
**Option B:** If this were the kidneys, it's incorrect because, like the liver, the kidneys also continue to grow after birth.
**Option D:** If this option were the heart, it's incorrect because, although the heart is functional at birth, it also undergoes significant growth, especially in response to the changes in circulation pattern after birth.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the **brain** reaches about 70% of its adult size by the age of 2 years, making it one of the earliest structures to approach its adult size. This rapid growth underscores the importance of early childhood development and care for optimal neurological outcome.

**Correct Answer:** Unfortunately, without the specific options provided, I'll use a hypothetical correct answer for illustration: **Correct Answer: C. Brain**