**Core Concept**
The question is testing the student's knowledge of the causative agent of malaria, which is a protozoan parasite. Malaria is a vector-borne disease caused by the Plasmodium species, which infects humans through the bite of an infected female Anopheles mosquito.

**Why the Correct Answer is Right**
The Plasmodium species, such as Plasmodium falciparum, Plasmodium vivax, and Plasmodium malariae, are unicellular eukaryotic parasites that belong to the phylum Apicomplexa. They have a complex life cycle that involves both human and mosquito hosts. The parasite invades the erythrocytes (red blood cells) of the human host, causing the characteristic symptoms of malaria, such as fever, chills, and anemia. The parasite multiplies within the erythrocytes, leading to their destruction and release of new parasites into the bloodstream.

**Why Each Wrong Option is Incorrect**
* **Option B:** While mosquitoes are the vectors that transmit malaria, they are not the causative agents themselves. Mosquitoes become infected with the Plasmodium parasite when they feed on the blood of an infected human.
* **Option C:** Bacteria are not the causative agents of malaria. Bacterial infections, such as typhoid fever, are caused by different types of bacteria, such as Salmonella Typhi.
* **Option D:** Viruses are not the causative agents of malaria. Viral infections, such as influenza, are caused by different types of viruses, such as the influenza virus.

**Clinical Pearl / High-Yield Fact**
The Plasmodium parasite is transmitted through the bite of an infected female Anopheles mosquito, which is the primary vector of malaria. The mosquito becomes infected when it feeds on the blood of an infected human, and it can transmit the parasite to other humans through its bite. This is why malaria is often associated with areas with high mosquito populations and poor sanitation.

**✓ Correct Answer: A. Protozoa**