**Core Concept**
The diagnosis of Ascaris lumbricoides, a large intestinal nematode, relies on clinical presentation, laboratory findings, and sometimes direct visualization. The parasite's lifecycle involves ingestion of eggs, which hatch into larvae in the small intestine, and subsequent migration to the liver and lungs before returning to the intestine as adult worms.

**Why the Correct Answer is Right**
The correct answer is based on the specific method of diagnosis, which involves the microscopic examination of fecal samples to identify the parasite's characteristic eggs. The eggs of Ascaris lumbricoides are large, yellowish, and have a characteristic bipolar plug. The eggs are usually identified in a direct fecal smear or in a concentration technique such as the formalin-ether concentration method.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify a method of diagnosis. While clinical presentation can suggest the presence of Ascaris, it is not a specific diagnosis.
* **Option B:** This option is incorrect because serological tests are not commonly used for the diagnosis of Ascaris infections.
* **Option C:** This option is incorrect because imaging studies, such as X-rays or ultrasound, may show the presence of worms, but they are not the primary method of diagnosis.

**Clinical Pearl / High-Yield Fact**
The eggs of Ascaris lumbricoides can remain viable in the environment for up to 3 years, emphasizing the importance of proper sanitation and hygiene practices in preventing the spread of this infection.

**Correct Answer: D. Fecal examination to identify eggs.**