**Core Concept**
The question tests the student's knowledge of the **sympathetic nervous system** and **catecholamine** production. Norepinephrine is a crucial **neurotransmitter** involved in the body's **fight-or-flight response**.

**Why the Correct Answer is Right**
Norepinephrine is primarily produced in the **locus coeruleus** of the brain and in the **adrenal medulla**. The adrenal medulla is the inner part of the **adrenal gland**, and it is responsible for producing **catecholamines** like norepinephrine and **epinephrine**. The production of norepinephrine involves the enzyme **tyrosine hydroxylase**, which converts **tyrosine** into **L-DOPA**.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while the adrenal gland does produce norepinephrine, it is not the only source.
**Option B:** Incorrect as it is not a primary source of norepinephrine.
**Option C:** Incorrect because it is not directly related to norepinephrine production.

**Clinical Pearl / High-Yield Fact**
Remember that norepinephrine plays a critical role in **hypertension** and **cardiovascular disease**. Its dysregulation can lead to various clinical conditions.

**Correct Answer:** D. Adrenal medulla.