- The primary source of NADPH required for fatty acid biosynthesis is the Pentose phosphate pathway.
- This biochemical pathway plays a crucial role in providing the reducing power necessary for anabolic reactions, such as the synthesis of fatty acids.

- During fatty acid biosynthesis, NADPH acts as a reducing agent, facilitating the conversion of acetyl-CoA into long-chain fatty acids.
- While glycolysis primarily generates ATP and pyruvate, it does not directly produce significant amounts of NADPH.
- The citric acid cycle focuses on energy production through NADH and FADH2, rather than NADPH generation. 
- Beta-oxidation, conversely, is involved in fatty acid degradation, producing NADH and FADH2 rather than NADPH.

- The Pentose phosphate pathway specifically generates NADPH through its oxidative phase, where glucose-6-phosphate is converted into ribulose-5-phosphate.
- This reaction involves the enzyme glucose-6-phosphate dehydrogenase, which reduces NADP+ to NADPH, making this pathway essential for biosynthetic processes.

Hence, the correct answer is the Pentose phosphate pathway, as it primarily supplies the NADPH needed for fatty acid biosynthesis.

Reference: Lehninger Principles of Biochemistry, 7th Edition, Chapter 22/Fatty Acid Metabolism