The elements whose acids can turn litmus solution red are:
A.Lithium and sodium
B.Copper and potassium
C.Carbon and hydrogen
D.Phosphorus and Sulphur

Hint:To answer the question the property of the color change of litmus solution must be known. Litmus solution turns red in acidic medium whereas it turns blue in alkaline medium. Now try to solve the question with this knowledge.

Complete step by step answer:
-Litmus is a natural indicator of pH of a solution. The aqueous solution of litmus is light purple color. When it comes to contact with acidic solution it becomes red and becomes blue when a basic solution is added.
-Among the four options given here, first (lithium and sodium) and second (copper and potassium) are the metal elements and they do not produce acid.
-The third option contains carbon and hydrogen which have oxides like $\text{C}{{\text{O}}_{\text{2}}}$ and water (${{\text{H}}_{\text{2}}}\text{O}$) which are neutral in nature. So, there will be no color change against litmus solution.
-Now for the given fourth option, it is clearly shown that both phosphorus and sulphur are non-metal and their oxides are acids namely phosphoric acid (${{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{4}}}$) and sulphuric acid (${{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}$).

-So the correct option is D.

Note:
-Litmus is a blue dye extracted from lichens. Most of the lichen plants are the thallophytes which contain litmus. By the crushing and grinding of lichens litmus powder is obtained. The water solution of litmus used over the pH 4.5 to 8.3 to detect a solution is acidic or basic. Acidic solution gets red in contact with light purple litmus solution whereas the basic solution gets blue in contact with light purple litmus solution.
-Non-metallic oxides are the acidic oxides which turn litmus solution into red.