Indicators are used for identifying acids and bases...
Lemon is sour in test and it is acidic in natuare...
so it is not considered as an indicator....
hence correct answer is D.

Explanation:

Indicators:
Indicators are substances that can be used to indicate the presence or absence of certain properties or substances. They are commonly used in chemistry and biology experiments to determine the pH level, the presence of specific ions, or the completion of a chemical reaction. They can be natural or synthetic compounds that change color or undergo some other visible change in the presence of certain conditions.

Options:
a) China rose: China rose, also known as Hibiscus rosa-sinensis, is a flowering plant. It is not commonly used as an indicator in scientific experiments. Although some flowers can change color in response to changes in pH or other factors, China rose is not specifically known for its indicator properties.

b) Onion: Onion is a vegetable commonly used in cooking. It is not an indicator and does not change color or undergo any visible change in the presence of specific conditions.

c) Clove: Clove is a spice derived from the flower buds of the Syzygium aromaticum tree. It is not commonly used as an indicator. Clove does not change color or undergo any visible change in the presence of specific conditions.

d) Lemon: Lemon is a citrus fruit commonly used in cooking and as a flavoring agent. Unlike the other options, lemon can act as an indicator. Lemon juice is acidic and can change the color of certain indicators, such as litmus paper, from blue to red.

Conclusion:
Out of the given options, lemon is the only one that can be used as an indicator. China rose, onion, and clove do not possess indicator properties. Therefore, the correct answer is option 'D' - Lemon.