Option 1 -> Nitric Acid is a strong corrosive acid used in manufacturing fertilizers and explosives, not in bathroom cleaning or digestion.
Option 2 -> Boric Acid is a weak acid used as an antiseptic and insecticide, not commonly found in bathroom cleaners or gastric juices.
Option 3 -> Hydrochloric Acid is used in toilet cleaners to remove mineral deposits and lime scale, and is the primary acid in stomach gastric juices.
Option 4 -> Phosphoric Acid is used in soft drinks and rust removal, but not typically as bathroom cleaner or in gastric juices.
Hence, Option 3: Hydrochloric Acid -> It serves dual purposes: commercially, it's used in bathroom/toilet cleaners to dissolve tough stains, mineral deposits, and lime scale; biologically, it's secreted by the stomach lining (0.5% concentration) to break down food, kill bacteria, and activate pepsinogen into pepsin for protein digestion -> correct