The correct answer is option 4, sporozoite. Sporozoites are the infective stage of the malarial parasite. After a mosquito carrying the parasite bites a human, sporozoites are injected into the bloodstream. From there, they travel to the liver where they infect liver cells. Inside the liver cells, sporozoites mature into a different stage called schizonts and multiply. The schizonts then release merozoites into the bloodstream, which invade red blood cells and continue to multiply. Eventually, the infected red blood cells burst, releasing more merozoites. These merozoites can infect new red blood cells, causing the symptoms of malaria. So, while the other options may represent different stages of the malarial parasite, it is the sporozoite stage that is responsible for the initial infection.