A gas turns moist red litmus blue but has no effect on dry red litmus. What is the correct conclusion about the gas?

Correct answer: It forms a basic solution in moisture

A red litmus paper turns blue when a basic medium is present. The gas shows this effect only when moisture is available, because water enables the gas to dissolve and produce ions responsible for basicity. Ammonia is the familiar example: it dissolves in water and forms an alkaline solution. Hence option A is correct; the dry paper lacks the aqueous medium needed for the colour change.