Concept: ACID: It is an ion or a molecule that is capable of donating a hydrogen ion i.e. proton. It has a sour taste. It is corrosive in nature. it changes the colour of litmus, from blue to red. Its name come from the Latin word "acidus". which means "sour" and refers to the sour taste of many acids. Its ph value range is between 1 to 7. example: HCL, H2SO4 etc BASE: ​The base gives off hydroxyl ion when dissolved in water. it changes red litmus paper to blue It has a bitter taste Its pH value range is between 7 to 14. Example: NaOH, KOH etc. Indicator: It is something that gives information or shows a sign. Examples of Indicators of acid and base are: Litmus paper Methyl orange Phenolphthalein. Red Litmus Blue Litmus Acidic solution Stays red Turns red Neutral solution Stays red Stays blue Alkaline solution Turns blue Stays blue Indicator Acidic Neutral Alkaline Methyl orange Red Neutral Yellow Phenolphthalein Colourless Colourless Pink Explanation: Sometimes indicator's colour cannot be changed for both acid and bases If an indicator gives a colour change for an acid then it can give a colour change for the base as well. The colour change depends on the type of indicator as some indicators show different colours. Hence option D is the correct answer. Additional Information [ alt="F5 Savita Teaching 18-4-22 D1" src="//storage.googleapis.com/tb-img/production/22/04/F5_Savita_Teaching__18-4-22_D1.png" style="width: 451px; height: 172px;">