**Core Concept**
Penicillins are a class of beta-lactam antibiotics that work by inhibiting bacterial cell wall synthesis. However, their effectiveness is limited by their narrow spectrum of activity, particularly against gram-negative bacteria, which have a thicker peptidoglycan layer and an outer membrane that restricts the entry of penicillins.

**Why the Correct Answer is Right**
The best gram-negative spectrum among penicillins is attributed to **Carbenicillin (C)**, a broad-spectrum penicillin that is resistant to beta-lactamase degradation. Carbenicillin achieves this through its unique structure, which allows it to maintain its antibacterial activity even in the presence of beta-lactamase enzymes produced by gram-negative bacteria. This property enables Carbenicillin to effectively target a wide range of gram-negative pathogens, including Pseudomonas aeruginosa.

**Why Each Wrong Option is Incorrect**
**Option A:** **Ampicillin** has a narrower spectrum than Carbenicillin and is more susceptible to beta-lactamase degradation, making it less effective against gram-negative bacteria.

**Option B:** **Penicillin G** has a very narrow spectrum and is primarily effective against gram-positive bacteria, with limited activity against gram-negative bacteria.

**Option D:** **Oxacillin** is a penicillinase-resistant penicillin but has a narrower spectrum than Carbenicillin and is more effective against gram-positive bacteria.

**Clinical Pearl / High-Yield Fact**
When choosing a penicillin for treating gram-negative infections, consider the specific pathogen and its susceptibility to beta-lactamase enzymes, as well as the presence of any resistance mechanisms.

**Correct Answer:** C. Carbenicillin.