Question

Acid turn blue litmus:
A.Green
B.Red
C.Yellow
D.orange

Answer 1

Hint: Blue litmus paper turns red under acidic conditions and red litmus paper turns blue under basic conditions. With colour change occurring over the pH range 4.5 to 8.3 at ${25^0}C$. Neutral litmus paper is purple.

Complete step by step answer:
Litmus is a water soluble mixture of different dyes extracted from lichens, especially roccella tinctoria. It is often absorbed onto filter paper to produce one of the oldest forms of pH indicators, used to test materials for acidity.
Blue litmus paper turns red under acid conditions and red litmus paper turns blue under basic conditions ( i.e. alkaline)
The pigment in blue litmus reacts with ${H^ + }$ ions and changes chemically so that bonds are tuned to reflect a longer wavelength of light to appear red to your eyes. The pigment is absorbing the blue to green wavelengths when it reacts with the free hydrogen ions in acids. Otherwise the same pigment in base reacts differently with $O{H^ - }$ ions and becomes ‘tuned’ to reflect the short wavelength to appear blue. Now it absorbs the green and red wavelengths having reacted with hydroxyl ions in the base.
So the answer is (B) red.

Note:
Red litmus test in acid conditions. Red stays red and blue stays blue in neutral conditions. Whereas blue litmus stays blue in basic conditions