**Core Concept**
Relapse in malaria occurs due to the reactivation of latent hypnozoites of Plasmodium vivax and Plasmodium ovale, which are not affected by the initial antimalarial treatment.

**Why the Correct Answer is Right**
The correct answer is related to the concept of hypnozoite reactivation. Hypnozoites are dormant stages of the parasite that can remain in the liver for months or even years, leading to relapse. The only antimalarial drugs that can target hypnozoites are primaquine and tafenoquine, which are used in the treatment of relapse cases. The other options do not directly relate to the mechanism of relapse in malaria.

**Why Each Wrong Option is Incorrect**
**Option A:** Chloroquine is effective against the erythrocytic stages of the parasite but does not target hypnozoites, making it ineffective against relapse.
**Option B:** Artemisinin-based combination therapies (ACTs) are effective against the blood stages of the parasite but do not prevent relapse.
**Option C:** Sulfadoxine-pyrimethamine (SP) is used in the treatment of uncomplicated malaria but does not target hypnozoites.

**Clinical Pearl / High-Yield Fact**
It's essential to note that primaquine is contraindicated in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency, as it can cause hemolysis.

**Correct Answer: D. Primaquine.