## Core Concept
Yersinia pestis, the causative agent of plague, exhibits specific characteristics that are crucial for its identification. It is a type of bacterium that can be described in terms of its Gram staining properties, motility, and morphology.

## Why the Correct Answer is Right
Yersinia pestis is a **Gram-negative** bacterium. This characteristic is critical for its identification and is a key feature that distinguishes it from Gram-positive bacteria. The correct description of Y. pestis includes being **Gram-negative (b)**, having a **coccobacilli** morphology (e), and being **non-motile (d)** at 37°C, which is a key characteristic for its identification.

## Why Each Wrong Option is Incorrect
- **Option A: Gram +ve** - This is incorrect because Y. pestis is Gram-negative, not Gram-positive.
- **Option C: Motile** - This is incorrect because, at human body temperature (37°C), Y. pestis is non-motile, although it can exhibit motility at lower temperatures.

## Clinical Pearl / High-Yield Fact
A crucial clinical pearl is that Y. pestis is transmitted to humans through the bites of infected fleas, direct contact with infected animals, or through respiratory droplets from person to person in the case of pneumonic plague. Rapid identification and treatment are critical.

## Correct Answer: B.