An aqueous solution of a salt turns red litmus blue. From which pair is this salt more likely to be formed?

Correct answer: A weak acid and a strong base

Red litmus turning blue indicates that the salt solution is basic. A salt formed from a weak acid and a strong base commonly gives a basic solution because the anion of the weak acid hydrolyses water and produces hydroxide ions; sodium carbonate is a familiar example. A strong-acid/weak-base salt is generally acidic, while a strong-acid/strong-base salt is approximately neutral. Hence A is correct.