The infectious stage of malaria is (d) Sporozoite. Sporozoites are the form of the malaria parasite transmitted to humans through the bite of infected mosquitoes. When an infected mosquito bites a person, it injects sporozoites into the bloodstream. From there, sporozoites travel to the liver, where they invade liver cells and multiply, initiating the infection cycle. After replication in the liver, sporozoites develop into merozoites, which are released into the bloodstream, infecting red blood cells and causing symptoms of malaria. Thus, while merozoites are responsible for the symptomatic phase, sporozoites initiate the infection by entering the host’s bloodstream.