**Core Concept**
The question pertains to the microscopic features of parasites, specifically the morphology of their eggs. In medical parasitology, identifying the shape and characteristics of eggs is crucial for diagnosing infections. The correct answer will highlight a specific type of egg morphology that is characteristic of a particular parasite.

**Why the Correct Answer is Right**
Barrel-shaped eggs are typically seen in the parasite *Taenia saginata*, also known as the beef tapeworm. The eggs of *Taenia saginata* are oval and have a characteristic barrel shape due to the presence of six prominent ridges. These ridges are caused by the cuticula, a thin layer of protein that covers the eggshell. When viewed under a microscope, the barrel-shaped eggs of *Taenia saginata* are about 30-35 micrometers in length and 15-20 micrometers in width.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because barrel-shaped eggs are not characteristic of *Toxocara* species, which have spherical or slightly elongated eggs.
**Option B:** This option is incorrect because *Ascaris lumbricoides* eggs are typically large, yellowish, and have a smooth outer shell, but they are not barrel-shaped.
**Option C:** This option is incorrect because *Echinostoma* species have eggs that are usually smaller and have a distinctive polar thickening.

**Clinical Pearl / High-Yield Fact**
When examining stool samples for parasites, it's essential to note that the morphology of eggs can vary depending on the parasite species. A barrel-shaped egg is a key characteristic of *Taenia saginata* eggs, and identifying this feature can aid in the diagnosis of beef tapeworm infections.

**Correct Answer: D. Taenia saginata.