The correct answer is (a) Pyrethrum. Pyrethrum is a natural insecticide that is extracted from the dried flowers of chrysanthemums. It is effective against a wide range of insects, including mosquitoes. Pyrethrum is often used in mosquito repellents, as it is safe for humans and pets when used as directed.
Rotenone is a natural insecticide that is extracted from the roots of the derris plant. It is effective against a wide range of insects, including mosquitoes. However, rotenone is also toxic to fish and other aquatic life, so it should not be used near water.
Ephedrine is a stimulant that is used to treat asthma and other respiratory conditions. It is not effective against mosquitoes.
Therefore, the correct answer is (a) Pyrethrum.