**Core Concept**
The underlying principle being tested is the life cycle of *Toxoplasma gondii*, a parasitic protozoan. *Toxoplasma gondii* has a complex life cycle involving multiple stages and hosts. The infective form is crucial for initiating infection in humans and other intermediate hosts.

**Why the Correct Answer is Right**
The correct answer is related to the stage of *Toxoplasma gondii* that can initiate infection. This stage is highly resistant to environmental stressors, allowing it to survive outside a host. The mechanism of infection involves the ingestion of this resistant form, which then excysts in the intestine, releasing the infectious parasites.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not represent the primary infective form of *Toxoplasma gondii*.
**Option B:** Similarly, this option is not the infective form that is typically associated with initiating new infections.
**Option D:** This option is also incorrect as it does not accurately describe the infective stage of the parasite.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the infective form of *Toxoplasma gondii* is highly resilient and can be found in undercooked meat and contaminated water, making it a significant public health concern, especially for pregnant women and immunocompromised individuals.

**Correct Answer:** D. Oocyst.