**Core Concept**
The infectious stage of Plasmodium, the causative agent of malaria, is a critical aspect of the parasite's life cycle. This stage involves the transmission of the parasite from the human host to the mosquito vector, which is essential for the parasite's survival and propagation.

**Why the Correct Answer is Right**
The infectious stage of Plasmodium is the sporozoite stage, which is the form of the parasite that is transmitted to the human host through the bite of an infected mosquito. Sporozoites are injected into the host's bloodstream and travel to the liver, where they invade hepatocytes and begin to multiply. This stage is essential for the initiation of the malaria infection in humans. The sporozoite stage is characterized by the presence of a polar ring, a prominent organelle that is involved in the invasion of host cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the trophozoite stage is not the infectious stage of Plasmodium. Trophozoites are the feeding stage of the parasite, which occurs in the erythrocytes (red blood cells) of the human host.
* **Option B:** This option is incorrect because the schizont stage is not the infectious stage of Plasmodium. Schizonts are the stage of the parasite that occurs in the erythrocytes of the human host, where they multiply and form merozoites.
* **Option C:** This option is incorrect because the gametocyte stage is not the infectious stage of Plasmodium. Gametocytes are the stage of the parasite that occurs in the erythrocytes of the human host, where they are ingested by the mosquito vector.

**Clinical Pearl / High-Yield Fact**
The sporozoite stage of Plasmodium is the only stage of the parasite that can be transmitted to the human host through the bite of an infected mosquito. This is why mosquito control measures, such as eliminating standing water and using insecticides, are critical in preventing the spread of malaria.

**Correct Answer: C. Gametocyte stage is the sexual stage of Plasmodium that is ingested by the mosquito vector, but it is not the infectious stage of Plasmodium. The correct answer is not listed here, but the correct answer would be D. Sporozoite stage.**