**Core Concept**
The question tests the knowledge of the largest and longest worms in the medical and biological context. The longest worms are a part of the phylum Annelida, which includes earthworms and leeches.

**Why the Correct Answer is Right**
The longest worm is the bootlace worm (Lineus longissimus), a type of marine nemertean worm that can grow up to 55 meters (180 feet) in length. This remarkable length is due to the worm's unique body structure, which consists of a long, slender body with a series of segments that can be stretched and contracted to accommodate its environment.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a worm species known for its exceptional length.
**Option B:** This option is incorrect because it is not a known species of worm that holds the record for the longest length.
**Option C:** This option is incorrect because it is a common earthworm, which while long, is not the longest worm species.

**Clinical Pearl / High-Yield Fact**
The bootlace worm's remarkable length is an example of how some organisms have evolved to adapt to their environments in unique and fascinating ways. This knowledge can be useful for medical professionals who need to understand the diversity of life on our planet.

**Correct Answer: D. Bootlace worm (Lineus longissimus)**