## Core Concept
The question tests the knowledge of bacterial motility, which is an important characteristic used in the identification and classification of bacteria. Motility refers to the ability of a bacterium to move by itself, usually with the aid of flagella. Non-motile bacteria lack this ability.

## Why the Correct Answer is Right
Among the given options, *Klebsiella pneumoniae* is known to be non-motile. This bacterium is a Gram-negative, encapsulated, non-motile rod that can cause various infections, including pneumonia. The lack of motility is a key feature that distinguishes *Klebsiella pneumoniae* from other bacteria that have flagella and are motile.

## Why Each Wrong Option is Incorrect
* **Option A:** *Escherichia coli* is generally motile due to its flagella, which it uses for movement.
* **Option B:** *Proteus* species are also motile, known for their rapid swarming behavior on agar plates.
* **Option C:** *Salmonella* species are typically motile, using their flagella for movement, although some strains may exhibit variations.

## Clinical Pearl / High-Yield Fact
A useful clinical pearl is that *Klebsiella pneumoniae* is often associated with severe community-acquired pneumonia, especially in individuals with certain risk factors like alcoholism or diabetes. Its non-motility can be a helpful laboratory feature in its identification.

## Correct Answer: D. Klebsiella pneumoniae