## Core Concept
The question tests the understanding of antibiotic resistance mechanisms, specifically in gram-negative bacilli, and how they develop resistance to chloramphenicol, a broad-spectrum antibiotic.

## Why the Correct Answer is Right
The correct answer, **C. Acetylation of the antibiotic**, involves the modification of chloramphenicol itself, making it ineffective. Gram-negative bacteria can produce enzymes that acetylate chloramphenicol, thereby inactivating it. This mechanism is a common pathway for resistance to chloramphenicol.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because while efflux pumps do play a role in antibiotic resistance by pumping the antibiotic out of the cell, they are not the most important mechanism specific to chloramphenicol resistance in gram-negative bacilli.
- **Option B:** This option is incorrect as it does not specify a known primary mechanism of chloramphenicol resistance.
- **Option D:** This option is incorrect because, although enzymatic inactivation can occur through other means, acetylation (option C) is specifically notable for chloramphenicol.

## Clinical Pearl / High-Yield Fact
A key point to remember is that chloramphenicol resistance can significantly limit its use, especially in serious infections. The development of resistance through acetylation highlights the importance of judicious antibiotic use.

## Correct Answer: C. Acetylation of the antibiotic