**Core Concept**
The cephalic phase of gastric secretion is a reflex response to the sight, smell, or thought of food, leading to the release of digestive enzymes and acids in preparation for meal ingestion. This phase is mediated by the vagus nerve and involves the release of acetylcholine, which stimulates the parietal cells in the stomach to secrete gastric acid.

**Why the Correct Answer is Right**
During the cephalic phase, the release of acetylcholine stimulates the parietal cells to secrete gastric acid, which contributes to approximately 20-30% of total gastric secretion. This is because the cephalic phase is a preparatory phase that helps to increase the pH of the stomach in anticipation of food ingestion. The parietal cells release hydrogen ions (H+) into the gastric lumen, which is necessary for protein digestion.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the cephalic phase contributes a significant but not majority percentage of total gastric secretion.
* **Option B:** This option is incorrect because the cephalic phase is a preparatory phase and not a phase of actual food digestion.
* **Option C:** This option is incorrect because the gastric phase is the phase of actual food digestion and not the preparatory phase.

**Clinical Pearl / High-Yield Fact**
The cephalic phase of gastric secretion is often referred to as the "psychic" phase because it is triggered by the sight, smell, or thought of food. This phase is an important adaptation to help increase the pH of the stomach in anticipation of food ingestion.

**Correct Answer: A. 20-30% of total gastric secretion.**