If an aqueous solution of a substance tastes sour and turns blue litmus red, which category does it belong to?

Correct answer: Acid

An acid produces hydrogen ions, or hydronium ions in water, and commonly has a sour character. It changes blue litmus paper to red because the indicator responds to the acidic medium. Thus option A is correct. A base generally turns red litmus blue, a metal is a broad class of element rather than the indicated solution category, and a neutral gas does not explain both observations. In practice, chemicals must never be tasted; the statement describes properties only.