Which organism is responsible for causing malaria in humans?

Plasmodium

The correct answer is Option (4) - Plasmodium.

Plasmodium is a genus of parasitic protozoans transmitted through the bite of infected female Anopheles mosquitoes. Among the various species of Plasmodium, such as Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae, and Plasmodium ovale, P. falciparum is the most lethal and responsible for the majority of severe malaria cases.

Once introduced into the human bloodstream through a mosquito bite, the Plasmodium parasites invade and multiply within red blood cells, leading to the characteristic symptoms of malaria, including fever, chills, fatigue, and, in severe cases, potentially fatal complications.