**Core Concept**
The question is testing the student's knowledge of a specific type of parasitic worm that affects humans, specifically its common name.

**Why the Correct Answer is Right**
The correct answer is **Hymenolepis nana**, also known as the dwarf tapeworm. This is because Hymenolepis nana is a species of tapeworm that is the smallest known tapeworm to infect humans, typically measuring 2-6 mm in length. It is a cosmopolitan parasite that can be found worldwide and is often acquired through the ingestion of contaminated food or water.

**Why Each Wrong Option is Incorrect**
**Option A:** Taenia saginata is incorrect because it is also known as the beef tapeworm, which is a larger species of tapeworm that is typically acquired through the consumption of undercooked beef.
**Option B:** Diphyllobothrium latum is incorrect because it is also known as the fish tapeworm, which is a larger species of tapeworm that is typically acquired through the consumption of undercooked fish.
**Option C:** Echinococcus granulosus is incorrect because it is a species of tapeworm that causes echinococcosis, a disease characterized by the formation of cysts in various organs.

**Clinical Pearl / High-Yield Fact**
It is worth noting that Hymenolepis nana can be transmitted through the fecal-oral route, and can also be acquired through the ingestion of contaminated food or water. This highlights the importance of proper hygiene and sanitation in preventing the spread of this parasite.

**Correct Answer:** D. Hymenolepis nana