**Core Concept**
Gastric acid secretion is a tightly regulated process involving intrinsic and extrinsic factors. It is primarily mediated by parietal cells in the gastric mucosa, stimulated by gastrin, acetylcholine, and histamine, and inhibited by hormones like secretin and somatostatin. The total acid output reflects parietal cell function and is a key indicator of gastric acid production.

**Why the Correct Answer is Right**
Option A: Secretin inhibits gastric acid secretion by acting on the S cells in the antrum, reducing gastrin release and directly suppressing parietal cell activity.
Option B: H2 blockers (e.g., ranitidine, cimetidine) inhibit histamine H2 receptors on parietal cells, thereby reducing acid secretion.
Option C: Total acid output (measured via pH monitoring or acid titration) correlates directly with parietal cell mass and activity, as each parietal cell produces HCl in response to stimuli.
Thus, all statements are accurate and reflect fundamental physiology of gastric acid regulation.

**Why Each Wrong Option is Incorrect**
Option A: This statement is factually correct — secretin suppresses acid secretion, so the claim "secretion decreases with secretin" is valid.
Option B: H2 blockers do prevent acid release by blocking histamine-induced stimulation of parietal cells — this is a well-established mechanism.
Option C: Total acid output is a direct measure of parietal cell function and is used clinically to assess disease states like gastritis or hypersecretion.
All are correct, so "D. All" is the only valid choice.

**Clinical Pearl / High-Yield Fact**
In clinical practice, acid suppression with H2 blockers or PPIs is used to treat peptic ulcer disease and Zollinger-Ellison syndrome. Total acid output testing remains a gold standard for assessing parietal cell activity in research and diagnosis.

✓ Correct Answer: D. All