What toxicant can be used for mole treatment?

Zinc phosphide is the appropriate choice for mole treatment due to its effectiveness as a rodenticide. When ingested, zinc phosphide reacts with stomach acid to produce phosphine gas, which is toxic to rodents. This method works specifically for species like moles because it can be effectively applied in bait formulations that entice moles to consume it.

Formaldehyde is primarily used as a preservative and disinfectant and is not a suitable toxicant for mole control. Similarly, while arsenic is a toxic substance, it is typically not used for controlling moles, mainly due to its high toxicity to a wide range of species and the regulatory challenges associated with its use. Copper sulfate is also not used as a toxicant for mole treatment; it has limited use in pest control primarily for certain aquatic environments or as a fungicide, but it does not effectively target moles.

Thus, the use of zinc phosphide is both effective and specifically permitted for mole control, making it the best choice among the options provided.