Final answer:
The NADPH required for fatty acid synthesis primarily comes from the Pentose Phosphate Pathway, a metabolic pathway that also generates five-carbon sugars alongside NADPH.
Step-by-step explanation:
The NADPH for fatty acid synthesis primarily comes from the Pentose Phosphate Pathway (Option B). The Pentose Phosphate Pathway is a metabolic pathway that operates parallel to glycolysis, generating NADPH, which is essential for anabolic reactions, including the synthesis of fatty acids. This pathway also produces five-carbon sugars and ribose 5-phosphate, important for nucleotide synthesis. In contrast, the Krebs cycle (also known as the Citric Acid Cycle) produces NADH and FADH2, and glycolysis ends with the production of pyruvate.
The Electron Transport Chain uses these molecules to generate ATP, but it is not a source of NADPH. Therefore, for the purpose of fatty acid synthesis, the Pentose Phosphate Pathway is the principal provider of NADPH, which acts as a reducing agent in the process of synthesizing fatty acids on the fatty acid synthase (FAS) complex in eukaryotes.