The correct answer is option 2: Fumigants. Fumigants are a type of pesticide that is convenient for controlling stored grain pests.
Option 1, systemic pesticides, are absorbed into the plant and provide long-lasting control against pests that feed on the plant. However, this type of pesticide is not suitable for controlling stored grain pests as they do not target the pests directly.
Option 3, contact poisons, work by directly contacting the pests and killing them upon contact. While they can be effective for controlling some pests, they may not be practical for use in controlling stored grain pests as they may not come into contact with all pests in the storage area.
Option 4, stomach poisons, are ingested by the pests and then act on their digestive systems, killing them. While this type of pesticide can be effective for some pests, it may not be suitable for controlling all types of stored grain pests.
Therefore, among the given options, fumigants are the most convenient type of pesticide for controlling stored grain pests as they can penetrate and disperse in the storage area, effectively reaching pests in all stages of their life cycle and controlling them.