• Fungal Contamination and Mycotoxins:
• During storage in warm and humid tropical climates, finished aquafeeds and feed raw materials are vulnerable to mold growth (e.g., Aspergillus flavus, Penicillium, Fusarium), which produce lethal mycotoxins (aflatoxin $B_1$, ochratoxin, zearalenone).
• Propionic Acid and Propionates:
• Propionic acid (and its calcium/sodium salts) is the most widely utilized and effective antifungal agent (mold inhibitor) in compound feed manufacturing.
• It readily penetrates mold cell walls in un-ionized form, disassociates intracellularly to lower pH, disrupts cellular proton gradients, and inhibits metabolic enzymes.
• nalysis:
• (Propionic Acid): Correct.
• Options 1, 2, 4: Inorganic salts and strong mineral acids not used as safe organic mold inhibitors.

Correct option: (3) Propionic Acid

• Gastric Secretion:
• In vertebrate digestive systems, pepsinogen is the inactive zymogen precursor of the endopeptidase pepsin.
• It is secreted exclusively by the stomach:
• In mammals, by chief (zymogenic) cells.
• In teleost fishes possessing a stomach, by oxynticopeptic cells in the gastric mucosa, which simultaneously secrete both hydrochloric acid and pepsinogen.
• nalysis:
• (Stomach): Correct.
• (Small intestine): Secretes enterokinase, aminopeptidases, disaccharidases.
• (Pancreas): Secretes trypsinogen, chymotrypsinogen, procarboxypeptidase, and amylase.
• (Liver): Secretes bile salts.

Correct option: (1) Stomach

• Hierarchical Protein Structure:
• Primary Structure: Refers strictly to the linear sequence of amino acids joined together covalently by peptide bonds (amide linkages formed by condensation of $\alpha$-carboxyl and $\alpha$-amino groups).
• Secondary Structure: Regular local spatial conformations ($lpha$-helices, $eta$-pleated sheets) held together by hydrogen bonds.
• Tertiary & Quaternary Structure: Overall 3D spatial conformation stabilized by ionic bonds, hydrophobic interactions, van der Waals forces, and covalent disulfide bonds.
• nalysis:
• (Peptide bonds): Correct. The primary structure is established entirely by peptide linkages.

Correct option: (3) Peptide bonds