If a gas turns moist red litmus blue but has no effect on dry red litmus, what is the conclusion about the gas?

Correct answer: The gas forms a basic solution in moisture

A red litmus paper turns blue only when a basic medium is produced. The absence of an effect on dry paper shows that moisture is necessary for the gas to dissolve and generate the basic species or hydroxide ions responsible for the colour change. Ammonia is a familiar example: NH₃ dissolves in water and produces an alkaline solution. Thus option A is the correct conclusion.