**Core Concept**
Fatty acid synthesis is a critical anabolic process in which acetyl-CoA is converted into long-chain fatty acids, requiring reducing equivalents in the form of NADPH and acyl groups. This process is essential for lipid homeostasis and energy storage.

**Why the Correct Answer is Right**
The correct answer involves the reduction of acetyl-CoA into malonyl-CoA, which is catalyzed by acetyl-CoA carboxylase (ACC). This reaction requires NADPH as the reducing equivalent, which is generated from the pentose phosphate pathway. The malonyl-CoA is then used for elongation of fatty acids, a process that is tightly regulated by various enzymes and hormones.

**Why Each Wrong Option is Incorrect**
**Option A:** This is not a correct answer as it does not correctly identify the source of reducing equivalents for fatty acid synthesis.
**Option B:** This option is incorrect because it refers to a different metabolic pathway, where reducing equivalents are used for oxidation, not synthesis.
**Option C:** This is not a correct answer as it does not accurately describe the source of reducing equivalents for fatty acid synthesis.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the pentose phosphate pathway, particularly the non-oxidative phase, generates NADPH, which is crucial for fatty acid synthesis and other anabolic processes. A deficiency in this pathway can lead to impaired fatty acid synthesis and various metabolic disorders.

**Correct Answer:** D. NADPH.