**Core Concept**
Schistosoma (blood flukes) are parasitic flatworms that belong to the phylum Platyhelminthes. They are the causative agents of schistosomiasis, a significant public health problem in tropical and subtropical regions. These parasites have a complex life cycle that involves both aquatic and terrestrial environments.

**Why the Correct Answer is Right**
Schistosoma eggs are excreted in the feces of infected individuals and contaminate freshwater sources, such as rivers, lakes, and ponds. The eggs hatch into miracidia, which then infect specific snail species, including Biomphalaria and Bulinus. The infected snails release cercariae, which are the larvae that penetrate human skin during contact with contaminated water. The cercariae develop into schistosomula and eventually mature into adult worms that live in the blood vessels of the human host.

**Why Each Wrong Option is Incorrect**
* **Option A:** Schistosomes are not typically found in saltwater environments, such as oceans, which are more commonly associated with marine parasites like Anisakis.
* **Option B:** Terrestrial environments are not the primary habitat of Schistosoma, although they can survive for short periods outside the host.
* **Option D:** Soil is not a natural habitat for Schistosoma, as they require aquatic environments to complete their life cycle.

**Clinical Pearl / High-Yield Fact**
It's essential to note that Schistosoma infection is often asymptomatic in its early stages, but it can lead to significant morbidity if left untreated, including liver fibrosis, bladder cancer, and reproductive disorders.

**Correct Answer: C. Freshwater. Freshwater environments are the natural habitat of Schistosoma (blood flukes).**