Which of the following is an example of an inorganic insecticide?

Inorganic insecticides are chemicals that do not contain carbon-hydrogen (C-H) bonds; they typically consist of minerals or metal elements. The choice of sulfur as an inorganic insecticide is correct because sulfur is a naturally occurring element that has been used for centuries to control various pests and fungal diseases in horticulture and agriculture. It works by suffocating insects and disrupting their growth and metabolism.

In contrast, the other options listed are organic compounds. Chlorpyrifos and Malathion are organophosphate insecticides, which are based on carbon compounds and interfere with the nervous system of insects. Pyrethrin, derived from chrysanthemum flowers, is also an organic compound and functions by affecting the neural activity of insects. Thus, sulfur stands out as the only inorganic option in this context, making it the correct answer.