Which of the following is an inorganic poison used in pest control?

Arsenic is recognized as an inorganic poison used in pest control. Inorganic poisons are substances that do not contain carbon, and arsenic fits this definition as it is a metalloid element. In pest control, arsenic compounds have historically been used due to their toxicity to various pests, especially in the past for treating wood against wood-destroying organisms.

The other options listed, while effective in pest control, are organic compounds. Chlordane, malathion, and DDT are all organic pesticides. These compounds contain carbon and are synthetic or derived from natural sources, making them organic in nature. Each has specific applications and modes of action in pest management but does not meet the criteria for being classified as inorganic poisons.