Option 1: DDT is not used by farmers to destroy weeds. DDT is a pesticide that was widely used in the past for controlling insects, but its use has been greatly restricted due to its harmful effects on the environment and human health.

Option 2: Malathion is also not typically used by farmers to destroy weeds. It is an insecticide that is primarily used for controlling insects, especially in agricultural settings.

Option 3: Methyl bromide is a fumigant that is used to control pests and pathogens in the soil, but it is not specifically used for weed control. It is often used to prepare the soil before planting crops.

Option 4: The correct answer is 2, 4-D. This is a herbicide that is commonly used by farmers to control broadleaf weeds in various crops, including corn, wheat, and soybeans. It is highly effective in killing weeds while minimizing damage to the desired crops.

Overall, the correct answer is option 4 as 2, 4-D is the chemical that farmers typically use to destroy weeds in their fields.