In which situation will a neutralisation reaction certainly occur?

Correct answer: Mixing hydrochloric acid and sodium hydroxide

Neutralisation is the reaction of an acid with a base, usually producing salt and water. Hydrochloric acid supplies H⁺ ions and sodium hydroxide supplies OH⁻ ions; they combine as H⁺ + OH⁻ → H₂O, while Na⁺ and Cl⁻ form sodium chloride: HCl + NaOH → NaCl + H₂O. Mixing salt, sand, or sugar with water is dissolution or suspension, not an acid-base reaction. Thus A is certain.