Correct Option: D

Litmus is a water-soluble mixture of different dyes extracted from lichens, especially Roccellatinctoria. It is often absorbed onto filter paper to produce one of the oldest forms of pH indicator, used to test materials for acidity. Blue litmus paper turns red under acidic conditions and red litmus paper turns blue under basic (i.e. alkaline) conditions, with the colour change occurring over the pH range 4.5-8.3 at 25 degree C. Neutral litmus paper is purple. Litmus can also be prepared as an aqueous solution that functions similarly. Under acidic conditions the solution is red, and under basic conditions the solution is blue. The main use of litmus is to test whether a solution is acidic or basic. Wet litmus paper can also be used to test water-soluble gases; the gas dissolves in the water and the resulting solution colours the litmus paper. For instance, ammonia gas, which is alkaline, colours the red litmus paper blue.