**Core Concept**
Echinococcus granulosus is a parasitic tapeworm that causes cystic echinococcosis (CE), also known as hydatid disease. The definitive host is the final host where the parasite reaches maturity and reproduces.

**Why the Correct Answer is Right**
The definitive host for Echinococcus granulosus is the dog (Canis lupus familiaris), where the parasite reaches maturity in the small intestine. The adult worms attach to the intestinal mucosa and release eggs that are excreted in the feces. The eggs contain oncospheres that can infect intermediate hosts, such as sheep, cattle, and humans, where they develop into cystic structures.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because sheep (Ovis aries) are intermediate hosts, not definitive hosts, for Echinococcus granulosus.
**Option B:** Incorrect because cattle (Bos taurus) are also intermediate hosts, not definitive hosts, for Echinococcus granulosus.
**Option C:** Incorrect because humans (Homo sapiens) can be accidental intermediate hosts, but they are not the natural definitive hosts for Echinococcus granulosus.

**Clinical Pearl / High-Yield Fact**
Sheep are the primary intermediate hosts for Echinococcus granulosus, and the parasite's life cycle is closely linked to the sheep-dog interface in rural areas, highlighting the importance of controlling canine populations to prevent the spread of CE.

**Correct Answer:** D. Dog (Canis lupus familiaris)