**Core Concept**
The hexose monophosphate (HMP) pathway, also known as the pentose phosphate pathway (PPP), is a crucial metabolic pathway that generates NADPH and pentoses from glucose-6-phosphate. Transketolase is a key enzyme in this pathway, catalyzing the transfer of a two-carbon ketol group from one sugar to another.

**Why the Correct Answer is Right**
Transketolase requires thiamine pyrophosphate (TPP) as a coenzyme to facilitate the transfer of the two-carbon ketol group. This reaction is essential for the regeneration of ribulose-5-phosphate and for the production of NADPH, which is required for various cellular processes, including fatty acid synthesis and antioxidant defenses. The other enzymes listed do not require TPP for their catalytic activity. G6PD is involved in the oxidative phase of the HMP pathway, while 6-phosphogluconate dehydrogenase also participates in the oxidative phase. Transaldolase, on the other hand, participates in the non-oxidative phase of the HMP pathway and does not require TPP.

**Why Each Wrong Option is Incorrect**
**Option A:** G6PD is the enzyme responsible for the conversion of glucose-6-phosphate to 6-phosphogluconolactone, but it does not require TPP for its activity.
**Option B:** 6-phosphogluconate dehydrogenase is another enzyme involved in the oxidative phase of the HMP pathway, but it does not require TPP.
**Option D:** Transaldolase is an enzyme that participates in the non-oxidative phase of the HMP pathway, transferring a three-carbon dihydroxyacetone group from one sugar to another, but it does not require TPP.

**Clinical Pearl / High-Yield Fact**
Thiamine deficiency can lead to impaired transketolase activity, resulting in decreased NADPH production and increased oxidative stress. This can have significant clinical implications, including the development of beriberi and Wernicke-Korsakoff syndrome.

**✓ Correct Answer: C. Transketolase**