Litmus is a natural dye (extracted from lichens) commonly used as an acid–base indicator. It is available impregnated on paper strips in two forms — red litmus paper and blue litmus paper — and its colour change depends on the pH of the solution it contacts.

The general rule for litmus behaviour is:

• Acidic solutions turn blue litmus paper red, but have no effect on red litmus paper (it stays red).
• Basic (alkaline) solutions turn red litmus paper blue, but have no effect on blue litmus paper (it stays blue).
• Neutral solutions (like distilled water) cause no colour change in either paper.

Applying this to the three test tubes — dilute HCl (acid), NaOH solution (base), and distilled water (neutral):

The acidic test tube (HCl) is therefore uniquely identified by the observation that the red litmus paper shows no change while the blue litmus paper is turned red. This dual test (using both papers together) is important because using only one colour of litmus can create ambiguity — for example, a “no change” result alone could mean either “acid on red litmus” or “neutral substance on red litmus,” so both papers must be used to reach a definite conclusion.

The remaining descriptions correspond to different substances or are physically impossible: showing no change in both papers describes the neutral distilled water, not the acid; a solution turning red litmus blue is characteristic of the basic NaOH solution; and no real aqueous solution turns both red and blue litmus to the same blue colour, since red litmus turning blue is itself the marker of a base, while blue litmus staying blue simply reflects the absence of acidity — this combined description does not correspond to any of the three solutions in a way that identifies the acid.