**Core Concept**
The question requires knowledge of the pharmacological approach to controlling vector-borne diseases, specifically leishmaniasis, which is transmitted by sandflies. The correct insecticide should have a rapid onset of action, long-lasting effects, and minimal environmental impact.

**Why the Correct Answer is Right**
The insecticide of choice for controlling sandflies is Deltamethrin. It belongs to the pyrethroid class of insecticides, which act on the nervous system by inhibiting the voltage-gated sodium channels. This results in persistent activation of sodium channels, leading to repetitive nerve firing and eventual death of the insect. Deltamethrin is a potent and long-lasting insecticide, making it a suitable choice for controlling sandfly populations.

**Why Each Wrong Option is Incorrect**
- **Option A:** Pyrethrum is a natural insecticide derived from chrysanthemum flowers, but it has a short duration of action and is less effective against sandflies compared to Deltamethrin.
- **Option B:** Permethrin is another pyrethroid insecticide, but it is not as potent or long-lasting as Deltamethrin, making it a less suitable choice for controlling sandfly populations.
- **Option C:** Bendiocarb is an organophosphate insecticide that acts on acetylcholinesterase, but it is not typically used for controlling sandflies and has a narrower spectrum of activity compared to Deltamethrin.

**Clinical Pearl / High-Yield Fact**
When considering insecticides for vector control, it's essential to evaluate their efficacy, safety, and environmental impact. Deltamethrin is a good example of a pyrethroid insecticide that meets these criteria, making it a preferred choice for controlling sandfly populations.

**Correct Answer:** D. Deltamethrin