Which of the following is used in rocket fuels?

Correct answer: HNO₃ (nitric acid).

Explanation: Concentrated/fuming nitric acid is a powerful oxidizer historically used in liquid rocket propellants (e.g., red-fuming nitric acid with fuels like hydrazine/UDMH). The others are not standard rocket oxidizers:

• CH₃COOH (acetic acid): weak acid, not an oxidizer for rockets.
• H₂SO₄ (sulfuric acid): strong acid/dehydrating agent but not used as a rocket oxidizer.
• HCl (hydrogen chloride): corrosive gas/acid, not a rocket propellant oxidizer.