**Core Concept**
The question targets the mechanism of action of antibiotics, specifically the class that inhibits cell wall synthesis. This concept is crucial in understanding the pharmacodynamics of antibiotics, which is essential for effective treatment of bacterial infections.

**Why the Correct Answer is Right**
Penicillin is a beta-lactam antibiotic that works by inhibiting the synthesis of the bacterial cell wall. It binds to penicillin-binding proteins (PBPs) located inside the bacterial cell wall, which are essential for the cross-linking of peptidoglycan layers. This inhibition prevents the bacterial cell wall from forming, ultimately leading to cell lysis and death. The mechanism of action of penicillin is specific to gram-positive and gram-negative bacteria, making it a critical component in the treatment of various bacterial infections.

**Why Each Wrong Option is Incorrect**
**Option A:** Chloramphenicol is a bacteriostatic antibiotic that inhibits protein synthesis by binding to the 50S subunit of the bacterial ribosome, not by inhibiting cell wall synthesis.

**Option B:** Gentamicin is an aminoglycoside antibiotic that works by binding to the 30S subunit of the bacterial ribosome, thereby inhibiting protein synthesis. It does not affect cell wall synthesis.

**Option C:** Erythromycin is a macrolide antibiotic that also inhibits protein synthesis by binding to the 50S subunit of the bacterial ribosome. It is not involved in cell wall synthesis.

**Clinical Pearl / High-Yield Fact**
When treating bacterial infections, it's essential to consider the mechanism of action of the antibiotic and its spectrum of activity. Beta-lactam antibiotics like penicillin are critical in treating infections caused by gram-positive and gram-negative bacteria, but they may not be effective against bacteria that have developed resistance mechanisms.

**✓ Correct Answer: D. Penicillin**