Match the following: (i) red cabbage leaves, (ii) methyl orange, (iii) onion, clove; with (a) olfactory indicator, (b) acid-base indicator, (c) natural acid-base indicator.

The correct answer is D, (i) - (c), (ii) - (b), (iii) - (a). Red cabbage is a natural acid-base indicator, methyl orange is a synthetic acid-base indicator, and onion and clove are olfactory indicators. An indicator tells whether a solution is acidic or basic. Natural indicators come from plants: litmus from lichen, turmeric, red cabbage leaves and coloured petals such as hydrangea and petunia. Synthetic indicators made in the laboratory include methyl orange, which is red in acid and yellow in base, and phenolphthalein, which is colourless in acid and pink in base. Olfactory indicators change their smell instead of their colour; onion, vanilla and clove lose their smell in a basic solution. A is wrong because it calls methyl orange an olfactory indicator. B is wrong because it labels red cabbage as olfactory. C is wrong because it puts onion and clove under natural colour indicators. Exam tip: change of smell = olfactory (onion, clove, vanilla); change of colour = litmus, turmeric, red cabbage; lab-made = methyl orange, phenolphthalein.