**Core Concept**
Non-motile Clostridia refers to a group of bacteria within the genus Clostridium that lack the ability to move using flagella. This characteristic is significant in clinical and laboratory settings, as it affects the identification and classification of these microorganisms.

**Why the Correct Answer is Right**
The correct answer is related to the species Clostridium tetani, which is known for causing tetanus. Clostridium tetani is a non-motile, anaerobic, Gram-positive bacterium that produces a potent neurotoxin responsible for the tetanic symptoms. The lack of motility in C. tetani is due to the absence of flagella, which are essential for bacterial movement. This characteristic is crucial in distinguishing C. tetani from other Clostridium species.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect as it does not specifically refer to a non-motile Clostridium species. While some Clostridium species may be non-motile, this option is too vague and does not accurately represent the characteristic being tested.

**Option B:**
This option is incorrect as it refers to Clostridium difficile, a motile bacterium that is often associated with antibiotic-associated diarrhea and colitis. C. difficile is known for its ability to produce toxins that cause gastrointestinal symptoms.

**Option C:**
This option is incorrect as it does not accurately represent a non-motile Clostridium species. While some Clostridium species may be non-motile, this option is not a valid choice.

**Clinical Pearl / High-Yield Fact**
In the laboratory setting, the identification of non-motile Clostridia, such as Clostridium tetani, is often confirmed through the use of biochemical tests and molecular techniques, such as PCR. These methods can help differentiate C. tetani from other Clostridium species and confirm the presence of the tetanus toxin gene.

**Correct Answer: D. Clostridium tetani**