**Core Concept**
The question is testing the understanding of post-translational modification of enzymes and their role as cofactors in biochemical pathways. **Post-translational modification** refers to the process by which enzymes are modified after translation to increase their activity or alter their function. These modifications can include phosphorylation, ubiquitination, or modification of cofactors.

**Why the Correct Answer is Right**
The correct answer is **B. Flavin Adenine Dinucleotide (FAD)**. FAD is a cofactor that undergoes a post-translational modification to become **Flavin Mononucleotide (FMN)**. FMN is then used as a cofactor for various enzymes, including those involved in the electron transport chain and fatty acid oxidation. This modification is crucial for the proper functioning of these enzymes. **The modification of FAD to FMN increases its ability to participate in redox reactions**, allowing it to facilitate the transfer of electrons in various biochemical pathways.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the concept of post-translational modification of cofactors.
* **Option C:** This option is not a correct example of a cofactor that undergoes post-translational modification.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that post-translational modification of enzymes can significantly alter their activity and function. This concept is crucial in understanding various biochemical pathways and diseases that result from enzyme dysfunction.

**Correct Answer: B. Flavin Adenine Dinucleotide (FAD)**