**Core Concept**
The infective form of Ascaris is a crucial aspect of parasitology, as it determines the mode of transmission of this parasitic worm. Ascaris lumbricoides is a large intestinal nematode that causes ascariasis, a common helminthic infection worldwide.

**Why the Correct Answer is Right**
The infective form of Ascaris is the third-stage larva, which is contained within embryonated eggs. These eggs are ingested by humans, typically through contaminated food or water. Inside the human host, the eggs hatch in the duodenum, releasing the third-stage larvae, which then penetrate the intestinal wall and migrate to the lungs via the bloodstream. This migration phase is crucial for the establishment of the infection. The third-stage larvae are then coughed up and swallowed, returning to the small intestine, where they mature into adult worms.

**Why Each Wrong Option is Incorrect**
**Option A:** The first-stage larva is not the infective form of Ascaris; rather, it is the stage that hatches from the egg in the duodenum.
**Option B:** The adult worm is not infective; it is the stage that resides in the small intestine and causes clinical symptoms.
**Option C:** The fourth-stage larva is not the infective form; it is the stage that develops from the third-stage larva in the small intestine.

**Clinical Pearl / High-Yield Fact**
Ascaris lumbricoides can cause a range of clinical symptoms, including abdominal pain, diarrhea, and respiratory distress due to the migration of the larvae through the lungs.

**Correct Answer: C. The third-stage larva.**