Step 1: Understanding the Question:
This question tests knowledge of organophosphate insecticides and their toxicity.
Step 2: Detailed Explanation:
Phorate is a highly toxic organophosphate insecticide.
It is a systemic insecticide used as a soil application.
Chlorpyriphos (A): Moderately toxic.
Dichlorvos (B): Highly volatile, moderate toxicity.
Ethion (C): Less toxic than phorate.
Thus, phorate is the most toxic.
Step 3: Final Answer:
Thus, the most toxic organophosphate insecticide is phorate, which corresponds to option (D).
[0.5cm]