Schistosomiasis is an infectious disease caused by parasitic worms known as schistosomes. The disease is commonly spread in freshwater environments where certain species of snails act as intermediate hosts. The correct answer to the question of how Schistosomiasis is spread is "Cercaria." Here's a detailed explanation:

1. Understanding Schistosomiasis:
   • Schistosomiasis is a water-borne parasitic disease caused by the trematode worms of the genus Schistosoma.
   • The disease primarily affects tropical and subtropical regions, especially in areas with poor sanitation and access to clean water.
2. Transmission of Schistosomiasis:
   • The life cycle of the schistosome involves humans and freshwater snails. Humans are the definitive host, while snails act as the intermediate host.
   • When infected individuals urinate or defecate in freshwater, the eggs of the parasite reach the water, where they hatch into larvae known as miracidia.
   • These miracidia infect susceptible snails and undergo development in them, eventually transforming into another larval form called "cercariae."
3. Role of Cercaria:
   • Cercariae are free-swimming larval forms that emerge from the snail intermediate host.
   • These cercariae have the ability to penetrate the skin of humans upon contact with contaminated water.
   • Once inside the human body, they develop into adult worms and cause the disease.
4. Ruling Out Other Options:
   • Giardia: A microorganism causing giardiasis, primarily a water-borne intestinal disease, but unrelated to Schistosomiasis.
   • E. Coli: A bacterium that can cause foodborne illness but is unrelated to Schistosomiasis.
   • Amoeba: Parasitic protozoa causing amoebiasis, unrelated to Schistosomiasis.

Given the above explanation, cercariae are indeed the agents through which schistosomiasis is spread in humans. This makes "Cercaria" the correct choice.