Final answer:
The B5 position in fatty acid synthase is found on the phosphopantetheine group of the acyl carrier protein (ACP).
Step-by-step explanation:
The B5 position in fatty acid synthase is found on the phosphopantetheine group of the acyl carrier protein (ACP).
The ACP acts as a carrier for the growing fatty acid chain during the synthesis process.
The fatty acid chain remains linked to the ACP throughout the reactions until it is eventually released as an acyl-CoA.