**Core Concept**
The question relates to the concept of Insecticide Resistance and the use of Integrated Pest Management (IPM) strategies to control vector-borne diseases. Insecticide Resistance Pattern (IRP) classification is often used to determine the effectiveness of insecticides against a particular vector population.

**Why the Correct Answer is Right**
When the Insecticide Resistance Pattern (IRP) classification is API > 2, it indicates that the vector population is resistant to DDT. In such cases, alternative insecticides like malathion should be used for control measures. Malathion is a broad-spectrum organophosphate insecticide that is effective against a wide range of pests, including vectors like mosquitoes and flies. To ensure its effectiveness, malathion spray should be done at regular intervals, typically every 7-10 days, to prevent the development of resistance.

**Why Each Wrong Option is Incorrect**
**Option A:** Malathion spray should be done every 14 days. This is incorrect because frequent intervals may not be sufficient to control the vector population, leading to resurgence of the disease.
**Option B:** No need for malathion spray as DDT is still effective. This is incorrect because the vector population has developed resistance to DDT, making it ineffective.
**Option C:** Malathion spray should be done only once. This is incorrect because a single application may not be sufficient to control the vector population, leading to re-infestation.

**Clinical Pearl / High-Yield Fact**
To prevent the development of resistance, insecticides should be used in rotation, and alternative control measures such as biological control, cultural control, and physical control should be employed in conjunction with chemical control.

**Correct Answer: C. Malathion spray should be done every 7-10 days.**