**Core Concept**
The substantia nigra is a critical region within the midbrain that plays a pivotal role in motor control and reward-based learning. It is primarily composed of dopaminergic neurons, which utilize dopamine as their major neurotransmitter.

**Why the Correct Answer is Right**
Dopaminergic neurons in the substantia nigra release dopamine, a catecholamine neurotransmitter that regulates movement, motivation, and reward processing. The dopaminergic pathway, particularly the nigrostriatal tract, is essential for motor control and coordination. The degeneration of dopaminergic neurons in the substantia nigra is a hallmark of Parkinson's disease, leading to characteristic motor symptoms such as bradykinesia, tremors, and rigidity.

**Why Each Wrong Option is Incorrect**
**Option B:** Acetylcholine is the primary neurotransmitter in the basal forebrain and is involved in attention and memory. However, it is not the major neurotransmitter in the substantia nigra.
**Option C:** Noradrenaline is a key neurotransmitter in the locus coeruleus, primarily involved in stress response and arousal. It is not the major neurotransmitter in the substantia nigra.
**Option D:** GABA (Gamma-Aminobutyric Acid) is an inhibitory neurotransmitter found in various regions of the brain, including the substantia nigra. However, it is not the major neurotransmitter in this region.

**Clinical Pearl / High-Yield Fact**
The loss of dopaminergic neurons in the substantia nigra can lead to significant motor impairment, as seen in Parkinson's disease. This highlights the importance of preserving dopaminergic function and the potential therapeutic benefits of dopaminergic agonists and inhibitors in treating motor disorders.

✓ Correct Answer: A. Dopaminergic.