**Core Concept**
Organophosphate insecticides are a class of chemicals used to control pests and insects. They work by inhibiting acetylcholinesterase, an enzyme that breaks down acetylcholine, a neurotransmitter in the nervous system. This inhibition leads to an accumulation of acetylcholine, causing overstimulation of muscles and glands, ultimately resulting in the death of the insect.

**Why the Correct Answer is Right**
Fenthion (b) and Propoxur (d) are not organophosphate insecticides. Fenthion is a phosphorus-based compound, but it belongs to the class of organothiophosphate insecticides. Propoxur, also known as 2-(1-methylpropyl)-phenothiazine, is a carbamate insecticide that acts by inhibiting acetylcholinesterase, but it is not an organophosphate. Dieldrin (a) is an organochlorine insecticide, and Lindane (e) is also an organochlorine insecticide, so both are not organophosphates either.

**Why Each Wrong Option is Incorrect**
**Option A:** Dieldrin is an organochlorine insecticide, not an organophosphate.
**Option B:** Fenthion is a phosphorus-based compound and an organothiophosphate insecticide, but not an organophosphate.
**Option B:** Propoxur is a carbamate insecticide, not an organophosphate.
**Option D:** Propoxur is a carbamate insecticide, not an organophosphate.
**Option E:** Lindane is an organochlorine insecticide, not an organophosphate.

**Clinical Pearl / High-Yield Fact**
Organophosphate poisoning is a significant public health concern, particularly in developing countries where these chemicals are often used. Early symptoms of organophosphate poisoning include muscarinic effects such as salivation, sweating, and lacrimation, followed by nicotinic effects such as muscle weakness and paralysis.

**✓ Correct Answer: C. ade. Organophosphate insecticides are all except Diazinon and Propoxur.**