Which of the following was primarily used to control aphids?

Nicotine is correct as it has been historically used for controlling aphids effectively. This natural insecticide was derived from the tobacco plant and contains potent insecticidal properties that target the nervous system of insects, including aphids. Its effectiveness stems from its ability to disrupt normal signaling in the insect's nervous system, leading to paralysis and death.

While pyrethrum, derived from chrysanthemum flowers, is also effective against a wide range of insects, it is not specifically noted for aphid control as much as nicotine was in prior decades. DDT, a once-common pesticide, is more associated with broader agricultural applications and has been banned in many countries, including the U.S., due to environmental concerns and persistence in ecosystems. Organophosphates are widely used for various pest controls, but they are more synthetic chemicals that affect a broader range of insects rather than being specifically tailored for aphids.

In summary, nicotine's targeted action and historical usage in pest management make it the primary choice among the options listed for controlling aphids specifically.