**Core Concept**
Transketolase is a crucial enzyme in the pentose phosphate pathway, responsible for the transfer of a two-carbon ketol group from one sugar to another. This enzyme plays a vital role in the generation of NADPH and pentoses from glucose-6-phosphate.

**Why the Correct Answer is Right**
Transketolase requires thiamine pyrophosphate (TPP) as a cofactor to facilitate the transfer of the two-carbon ketol group. TPP is a derivative of vitamin B1 (thiamine) and is essential for the enzyme's catalytic activity. The thiazolium ring of TPP is responsible for the enzyme's activity, allowing it to participate in the rearrangement of sugars in the pentose phosphate pathway. This enzyme's requirement for TPP is critical for maintaining cellular redox balance and generating energy-rich molecules.

**Why Each Wrong Option is Incorrect**
**Option A:** FAD (flavin adenine dinucleotide) is a redox cofactor involved in various enzymatic reactions, but it is not required for transketolase activity. FAD is crucial for enzymes like succinate dehydrogenase and NADH dehydrogenase.
**Option C:** PLP (pyridoxal phosphate) is a coenzyme involved in amino acid metabolism, particularly in transamination reactions. It is not required for transketolase activity.
**Option D:** FMN (flavin mononucleotide) is a coenzyme involved in redox reactions, particularly in the electron transport chain. It is not required for transketolase activity.

**Clinical Pearl / High-Yield Fact**
Thiamine deficiency, which leads to a lack of TPP, can cause serious neurological damage and is associated with Wernicke-Korsakoff syndrome. Transketolase activity is often used as a diagnostic marker for thiamine deficiency in clinical settings.

✓ Correct Answer: B. TPP