**Core Concept**
Salivary secretion is unique among gastrointestinal secretions due to its almost exclusive control by the nervous system, which distinguishes it from other GI secretions that are primarily controlled by hormonal mechanisms. This nervous control is mediated through the autonomic nervous system, specifically the parasympathetic branch.

**Why the Correct Answer is Right**
The correct answer is correct because it is the neurotransmitter released by the parasympathetic nervous system that inhibits salivary secretion. The parasympathetic nervous system releases acetylcholine, which acts on muscarinic receptors in the salivary glands to decrease saliva production. This is why salivary secretion is significantly inhibited by the parasympathetic nervous system.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the parasympathetic nervous system, not the sympathetic nervous system, primarily controls salivary secretion. The sympathetic nervous system is involved in stress responses and would actually stimulate salivary secretion.

**Option B:** This option is incorrect because dopamine is not a primary neurotransmitter involved in the control of salivary secretion. Dopamine is involved in various other physiological processes, including movement and reward.

**Option C:** This option is incorrect because gastrin is a hormone involved in gastric acid secretion, not salivary secretion.

**Clinical Pearl / High-Yield Fact**
The parasympathetic nervous system's control of salivary secretion is an important consideration in various clinical scenarios, such as the use of anticholinergic medications, which can lead to dry mouth and other salivary gland dysfunction.

**Correct Answer: A. Acetylcholine.**