**Core Concept**
DDT (Dichlorodiphenyltrichloroethane) is an organochlorine insecticide used for controlling a wide range of pests. It functions primarily through a mechanism of action known as _nervous system disruption_, which leads to the insect's death. In the context of its mode of action, DDT can be classified into two main categories: _contact poison_ and _stomach poison_.

**Why the Correct Answer is Right**
DDT primarily acts as a _stomach poison_. When ingested by an insect, DDT is absorbed into the bloodstream and eventually reaches the nervous system. Once it reaches the nervous system, DDT interferes with the normal functioning of the sodium channels, which disrupts the normal electrical activity of the nervous system. This results in the death of the insect. The incorrect statement is that DDT is a contact poison. As a contact poison, the insect would die immediately upon contact with the insecticide, which is not the case with DDT.

**Why Each Wrong Option is Incorrect**
**Option B:** The residual effect of DDT can indeed last for several months, but stating it lasts for 18 months is an overstatement. The actual duration can vary depending on factors such as climate, soil type, and usage.
**Option C:** DDT does not immediately kill the prey. Instead, the insect ingests the insecticide, which is then absorbed into the bloodstream and eventually leads to its death.
**Option D:** Permethrin is a synthetic pyrethroid insecticide that indeed has a synergistic action with piperonyl butoxide (PBO). PBO inhibits the cytochrome P450 enzymes, which in turn enhances the insecticidal activity of permethrin.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the use of DDT has been largely phased out due to its toxicity and environmental persistence. As a result, it's crucial to focus on the use of safer and more targeted insecticides in public health campaigns.

**✓ Correct Answer: A. It is contact poison.**