Final answer:
Protectin (CD59) is the molecule that regulates complement function by directly inhibiting the formation of the membrane-attack complex (MAC), protecting cells from being lysed by the complement system.
Step-by-step explanation:
The question relates to the regulation of the complement system, particularly the inhibition of the membrane-attack complex (MAC) formation. The MAC is a part of the immune response that can target and kill pathogens by disrupting their osmotic balance. Among the listed options, Protectin (CD59) is the molecule that directly inhibits MAC formation. Neither Factor I, DAF (CD55), nor Factor H performs this specific function. DAF and Factor H regulate complement activation at earlier steps, while Protectin is a cell surface protein that binds to the C8 and C9 components of the complement, thus preventing the assembly of the fully functional MAC on the cell to which it is attached.