Acids are substances that have a sour taste and are capable of reacting with bases to form salts and water. They also have certain characteristic properties that help in identifying them. The given options describe some of these properties and we need to choose the correct one.

Explanation:

The correct answer is option 'D', which says that acids turn blue litmus red. Let's see why.

Litmus is a natural dye obtained from certain lichens. It is commonly used as an indicator to test the acidic or basic nature of a substance. Litmus paper is a small strip of paper that has been impregnated with litmus solution and then dried. When this paper comes in contact with an acidic or basic solution, it changes color, indicating the nature of the solution.

Acids have a pH value less than 7, which means they are acidic in nature. When an acid comes in contact with blue litmus paper, it turns the paper red. This happens because the acid reacts with the water present in the litmus paper and donates a hydrogen ion (H+) to it. This hydrogen ion changes the color of the litmus dye and turns it red.

Other options mentioned in the question are incorrect because:

a) Red litmus turns blue in the presence of a base. This happens because bases have a pH value greater than 7 and they react with the litmus dye in a different way than acids.

b) Methyl orange is another type of indicator that is commonly used to test the acidic or basic nature of a substance. However, it is not a universal indicator like litmus and has a specific range of pH values at which it changes color. In the presence of an acid, methyl orange develops a pink color, not brown.

c) This option is incorrect for the same reason as option 'b'. In the presence of an acid, methyl orange develops a pink color, not brown.

Therefore, the correct answer is option 'D' - Acids turn blue litmus red.

The acid substance is sour in taste when we check in litmus paper it turn blue to red so d is correct option