**Core Concept**
Malignant transformation in hydatid cysts is a rare but serious complication. The underlying principle involves the parasitic infection caused by *Echinococcus* species. *Echinococcus granulosus* is the most common species responsible for hydatid disease.

**Why the Correct Answer is Right**
The correct answer is related to the species of *Echinococcus* that causes the infection. *Echinococcus granulosus* is the primary species implicated in hydatid disease, which can lead to malignant transformation in rare cases. This species infects humans through the accidental ingestion of eggs, leading to the formation of hydatid cysts.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect as it does not correspond to the correct species.
**Option B:** Similarly, this option is incorrect as it is not related to the *Echinococcus* species.
**Option C:** This option is also incorrect as the correct answer is not provided here.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that hydatid disease can have severe complications if left untreated, including malignant transformation. Early diagnosis and treatment are essential to prevent such outcomes.

**Correct Answer:** D. Echinococcus granulosus.