Litmus is a natural dye extracted from lichens that is widely used as a simple acid-base indicator because it changes colour depending on the pH of the solution it is dipped into. It is commercially available as red litmus paper and blue litmus paper, and the colour change (or lack of one) reveals the nature of the test solution:

• Acids — solutions containing an excess of H⁺ ions, with pH below 7 — turn blue litmus paper red, while red litmus paper shows no visible change since it is already in its acid-stable colour.
• Bases — solutions containing an excess of OH⁻ ions, with pH above 7 — turn red litmus paper blue, while blue litmus paper remains unchanged.
• Neutral solutions — with pH equal to 7, such as pure water or a common salt solution — cause no colour change in either red or blue litmus paper.

Litmus is only a rough, qualitative indicator: it reveals whether a substance is acidic, basic, or neutral, but not how strong or weak it is. For a more precise reading across the pH scale, a universal indicator (which shows a graded range of colours) or an electronic pH meter is used instead.

Applying this to the given options, an acid is correctly identified by the fact that it turns blue litmus paper red. Turning red litmus paper blue is instead the characteristic behaviour of a base, not an acid, so that option is incorrect here. Turning blue litmus paper green is not a genuine litmus response at all — litmus only shows the two extremes of red or blue, never an intermediate colour like green. Simply stating "no change in red litmus" is true for acids but, on its own, does not distinguish an acid from a neutral solution, since red litmus also shows no change with a neutral substance; it is the combination of blue litmus turning red that specifically and unambiguously signals an acid.