**Core Concept**
The Lancefield classification is a system used to group beta-hemolytic streptococci based on the presence of specific carbohydrate antigens on the bacterial cell wall. This classification system helps in identifying and differentiating between various streptococcal groups, each with distinct clinical manifestations and epidemiological characteristics.

**Why the Correct Answer is Right**
Group B streptococcus (GBS), also known as Streptococcus agalactiae, is a gram-positive bacterium that is a leading cause of neonatal meningitis and sepsis. GBS is a significant pathogen in pregnant women, and vertical transmission during delivery can lead to severe infections in newborns. The correct answer highlights the unique association of Group B streptococcus with neonatal meningitis, a condition that requires prompt recognition and management to prevent morbidity and mortality.

**Why Each Wrong Option is Incorrect**
**Option A:** The Lancefield classification is based on the presence of specific carbohydrate antigens on the bacterial cell wall, not the M protein. The M protein is a surface protein found on Streptococcus pyogenes, which is responsible for the virulence and invasive disease in humans, but it is not the basis for the Lancefield classification.

**Option B:** Group G streptococci can be isolated from various human sources, including the respiratory tract, skin, and gastrointestinal tract. Therefore, this statement is incorrect.

**Option D:** While Group C streptococci can be isolated from various human sources, there is no specific association with vaginal flora, making this statement incorrect.

**Clinical Pearl / High-Yield Fact**
It is essential to note that Group B streptococcus is a common commensal organism in the human gut and can be isolated from asymptomatic carriers. However, in pregnant women, GBS colonization can pose a significant risk to the neonate, emphasizing the importance of routine screening and prophylaxis during pregnancy.

**✓ Correct Answer: C. Group ‘B’ causes neonatal meningitis**