**Core Concept**
The infective form of Ascaris is a crucial aspect of its life cycle, allowing it to survive outside a host and transmit the infection. The infective form of Ascaris is a specific developmental stage that enables the parasite to withstand environmental stress and maintain its viability.

**Why the Correct Answer is Right**
The infective form of Ascaris is the third-stage larva, also known as the filariform larva. This stage is responsible for invading the intestinal wall and initiating the infection. The filariform larva is characterized by its ability to penetrate the intestinal mucosa, allowing it to reach the bloodstream and eventually establish itself in the lungs. The presence of the filariform larva triggers an immune response, leading to the production of IgE antibodies and eosinophilia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the egg stage of Ascaris is not the infective form. Although eggs are the primary means of transmission, they are not the stage that invades the host.

**Option B:** This option is incorrect because the adult stage of Ascaris is not the infective form. Adults are the reproductive stage of the parasite, responsible for producing eggs that are excreted in the feces.

**Option C:** This option is incorrect because the second-stage larva is not the infective form. The second-stage larva is a developmental stage that occurs within the host, but it is not the stage that penetrates the intestinal wall.

**Clinical Pearl / High-Yield Fact**
In order to diagnose Ascaris infection, it is essential to identify the presence of the filariform larva in the stool or intestinal tissue. This can be accomplished through microscopic examination or serological testing.

**Correct Answer: C. Third-stage larva (filariform larva)**