**Core Concept**
Antimalarial drugs can be classified based on their mechanism of action and the stage of the malaria parasite life cycle they target. Slow-acting schizonticides are a type of antimalarial drug that inhibit the growth and division of the parasite schizonts, which are the tissue forms of the parasite.

**Why the Correct Answer is Right**
The correct answer is **Chloroquine**. Chloroquine is a slow-acting schizonticide because it requires several days to exert its full effect on the parasite schizonts. Chloroquine works by binding to the parasite's hemoglobin and interfering with the parasite's ability to digest it, ultimately leading to the death of the parasite. This process takes several days, which is why chloroquine is considered a slow-acting schizonticide.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided, but typically, fast-acting schizonticides like Artemisinin are not slow-acting.
* **Option B:** This option is not provided, but typically, drugs like Proguanil are fast-acting and not slow-acting schizonticides.
* **Option C:** This option is not provided, but typically, drugs like Mefloquine are fast-acting and not slow-acting schizonticides.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the efficacy of antimalarial drugs can vary depending on the stage of the malaria parasite life cycle and the specific drug used. Slow-acting schizonticides like chloroquine are often used in combination with other antimalarial drugs to enhance their efficacy and reduce the risk of resistance.

**Correct Answer: D. Chloroquine**