## **Core Concept**
The question tests knowledge of bacterial morphology and classification, specifically focusing on Gram-negative cocci. Gram-negative bacteria have a thin peptidoglycan layer in their cell wall and an outer membrane containing lipopolysaccharides. Cocci are spherical or oval-shaped bacteria.

## **Why the Correct Answer is Right**
Neisseria species are well-known Gram-negative cocci. They are commonly found in the human body and can be pathogenic, causing diseases such as gonorrhea (Neisseria gonorrhoeae) and meningitis (Neisseria meningitidis). These bacteria are typically diplococci, often appearing in pairs.

## **Why Each Wrong Option is Incorrect**
* **Option A:** While some *Staphylococcus* species can have varying Gram staining properties, they are generally Gram-positive cocci, not Gram-negative.
* **Option B:** *Streptococcus* species are also Gram-positive cocci, often found in chains, and are not considered Gram-negative.
* **Option D:** *Bacillus* species are Gram-positive rods, not cocci, and are therefore not relevant to this question.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that *Neisseria gonorrhoeae* and *Neisseria meningitidis* are two of the most clinically significant Gram-negative cocci, and their identification is critical for treating sexually transmitted infections and bacterial meningitis, respectively.

## **Correct Answer:** C. Neisseria.