**Core Concept**
Transketolase is an enzyme involved in the pentose phosphate pathway (PPP), a metabolic pathway that generates NADPH and pentoses from glucose-6-phosphate. This enzyme plays a crucial role in the non-oxidative phase of the PPP, facilitating the transfer of two-carbon units between sugars.

**Why the Correct Answer is Right**
Transketolase requires thiamine pyrophosphate (TPP) as a coenzyme to catalyze the transfer of two-carbon units between ketoses and aldoses. TPP is a derivative of vitamin B1 (thiamine) and is essential for the enzyme's activity. The reaction involves the formation of a covalent intermediate between the enzyme and the substrate, which is then rearranged to form the product. The presence of TPP allows transketolase to facilitate the complex rearrangement of carbon skeletons.

**Why Each Wrong Option is Incorrect**
**Option A:** Thiamine hydrochloride - While thiamine hydrochloride is a form of vitamin B1, it is not the coenzyme required by transketolase. Thiamine hydrochloride is the form of thiamine used as a dietary supplement, but it needs to be converted to thiamine pyrophosphate (TPP) to be active.
**Option B:** NAD+ - NAD+ is a coenzyme involved in many redox reactions, but it is not required by transketolase. Transketolase requires TPP as a coenzyme, not NAD+.
**Option C:** FAD - FAD is another coenzyme involved in redox reactions, but it is not required by transketolase. The enzyme's activity is dependent on TPP, not FAD.

**Clinical Pearl / High-Yield Fact**
A deficiency in thiamine (vitamin B1) can lead to a decrease in transketolase activity, which is a diagnostic marker for thiamine deficiency. This condition can cause beriberi, a disease characterized by muscle weakness, fatigue, and nerve damage.

**Correct Answer:** D. Thiamine pyrophosphate (TPP)