**Core Concept**
The question pertains to human development, specifically focusing on the growth and maturation of organs or structures in the fetus. Understanding which structures attain adult size before birth is crucial for appreciating normal development and potential pathological conditions.

**Why the Correct Answer is Right**
Unfortunately, without the specific options provided, it's challenging to give a precise explanation. However, typically, the **kidneys** and **brain** are examples of organs that reach a significant portion of their adult size by the time of birth, with the brain being nearly 70% of its adult size. The mechanism involves complex hormonal and genetic pathways that regulate growth and development.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific option, it's hard to comment, but generally, organs like the **liver** and **pancreas** continue to grow significantly after birth.
**Option B:** Similarly, without specifics, but structures like the **lungs** are not fully developed or at adult size before birth, as they start functioning and expanding only after birth.
**Option C:** Again, lacking the option, but **musculoskeletal** structures continue to grow throughout childhood and adolescence.
**Option D:** Without the option, but typically, the **reproductive organs** are not at adult size before birth.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the brain reaches a significant percentage of its adult size by birth, which is crucial for understanding neurological development and potential impacts of prenatal or perinatal insults.

**Correct Answer:** D. Brain