**Core Concept**
Hydatid disease, also known as echinococcosis, is a parasitic infection caused by the larval stage of certain tapeworm species. The disease is characterized by the formation of cysts in various organs, primarily the liver and lungs.

**Why the Correct Answer is Right**
The correct answer is a species of Echinococcus, specifically Echinococcus granulosus, which is the most common cause of hydatid disease. This parasite is transmitted through the fecal-oral route, where the eggs are ingested by a definitive host, usually a carnivore, and then released in the environment through feces. Humans become intermediate hosts when they ingest the eggs, leading to the development of cysts in the liver or other organs.

**Why Each Wrong Option is Incorrect**
**Option A:** Echinococcus multilocularis is a different species that causes alveolar echinococcosis, a more aggressive and potentially life-threatening form of the disease.
**Option B:** Taenia saginata, also known as the beef tapeworm, is a different parasite that causes a different type of tapeworm infection.
**Option C:** Toxocara cati is a parasite that causes visceral larva migrans, a different type of parasitic infection.

**Clinical Pearl / High-Yield Fact**
Hydatid disease can be diagnosed through imaging studies, such as ultrasound or CT scans, which show the characteristic cysts. Treatment typically involves surgical removal of the cysts or the use of antiparasitic medications.

**Correct Answer: D. Echinococcus granulosus**