## 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 bacteria to move spontaneously by means of flagella. Non-motile bacteria lack flagella or have a defective flagellar apparatus, rendering them unable to move on their own.

## Why the Correct Answer is Right
The correct answer, , refers to *Streptococcus pneumoniae*, which is a non-motile bacterium. This organism is a type of alpha-hemolytic streptococcus and a common cause of pneumonia, meningitis, and otitis media. *S. pneumoniae* does not possess flagella, which makes it incapable of movement.

## Why Each Wrong Option is Incorrect
* **Option A:** *Escherichia coli* is a motile bacterium due to the presence of peritrichous flagella, which allow it to move.
* **Option B:** *Salmonella Typhi* is also motile, possessing peritrichous flagella that facilitate its movement.
* **Option C:** *Vibrio cholerae* exhibits motility through a single polar flagellum.

## Clinical Pearl / High-Yield Fact
A useful clinical pearl to remember is that the presence or absence of motility can be used as a diagnostic clue in differentiating between certain bacterial species. For instance, in the laboratory, the motility of bacteria can be assessed using a motility medium or by observing the spread of growth on a solid medium.

## Correct Answer: D. Streptococcus pneumoniae