**Core Concept**
The question is testing the knowledge of the life cycle of Trichuris trichiura, a parasitic nematode that causes whipworm infection. The correct answer requires understanding the developmental stages of this parasite, specifically the fully embryonated egg.

**Why the Correct Answer is Right**
A fully embryonated egg of Trichuris trichiura contains the third-stage larvae. This is a critical stage in the parasite's life cycle, as it is the form that is infective to humans. The egg hatches into a first-stage larva, which then molts twice to become a third-stage larva. This third-stage larva is the form that is found in the fully embryonated egg and is capable of infecting humans upon ingestion.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the first-stage larva is the form that hatches from the egg, not the form that is present in the fully embryonated egg.
* **Option B:** This option is incorrect because the second-stage larva is an intermediate stage that occurs between the first-stage larva and the third-stage larva.
* **Option D:** This option is incorrect because the adult worm is the form that is found in the human intestine, not the form that is present in the fully embryonated egg.

**Clinical Pearl / High-Yield Fact**
It is worth noting that Trichuris trichiura eggs can be found in the feces of infected individuals, and the presence of these eggs in a stool sample is a key diagnostic feature of whipworm infection.

**Correct Answer:** C.