Hydrochloric acid is secreted by the
A. Paneth cells
B. Goblet cells
C. Chief cells
D. Parietal cells

Hint: HCl acid is the aqueous or water-based, solution of hydrogen chloride gas. It is also the main compound of gastric acid, an acid generated naturally in the stomach to help ingest food.

Complete Answer:
Let us examine the given options to find the correct answer.
Option A: Paneth cells are functional intestinal epithelial tissues that secrete sufficient antimicrobial lysozyme, proteins, and thus disrupting Paneth cell secretion can point to inflammatory disease. Therefore, this is an incorrect option.

Option B: Goblet cells are unicellular intra-epithelial mucin-concealing glands distributed within simple epithelia, such as columnar cuboidal, and pseudostratified epithelia. Their name matches to their shape, as they resemble a goblet, with their thin bases and broad apex. Therefore this is an incorrect answer.

Option C: Gastric chief cells are commonly found deep in the mucosal coating of the stomach wall. Chief cells discharge the zymogen (enzyme precursor) pepsinogen when excited by a variety of constituents including cholinergic movement from the vagus nerve and acidic state in the stomach. Therefore, this is an incorrect answer.

Option D: Parietal cells (also identified as oxyntic cells) are epithelial cells in the stomach that conceal hydrochloric acid and intrinsic agents. These cells are found in the gastric glands seen in the wall of the fundus and cardia portions of the stomach. Therefore this is the correct answer.

Thus, option D is the correct answer.

Note: Parietal cells conceal acid in response to three kinds of stimuli:
- Histamine excites histamine receptors (most notable contribution).
- Acetylcholine, from parasympathetic movement via the enteric nervous system, and vagus nerve exciting Muscarinic acetylcholine receptor.
- Gastrin, exciting Cholecystokinin B receptors.