Which type of pesticide must be ingested by the target organism to be effective?

The correct choice is stomach poison because this type of pesticide is designed to be effective when ingested by the target organism. Stomach poisons function by disrupting the digestive system of pests after they consume the pesticide. This mechanism usually involves the poison causing physiological damage or impairment within the gut, leading to the pest's death.

Stomach poisons are specifically formulated to target the feeding behavior of pests, making it essential for the organism to eat the pesticide for it to work. This is in contrast to contact poisons, which act on contact with the organism's body, and systemic poisons, which are absorbed into the plant and affect pests that consume plant tissues. Inhalation poisons rely on the organism breathing in the pesticide, which is not how stomach poisons function.