Final answer:
Proteins with the C-terminal sequence (-KKKKKXXC) are most likely attached to the membrane by the cysteine residue forming a covalent bond with a membrane lipid.
Step-by-step explanation:
The most likely way in which the C-terminal sequence attaches these proteins to the membrane is through the cysteine residue being covalently attached to a membrane lipid. This is because the sequence mentioned (-KKKKKXXC) contains a cysteine at the end, which is known to form thioester linkages with the fatty acid chains of lipids. This covalent linkage is a strong form of attachment within the hydrophobic interior of the membrane. Integral membrane proteins are anchored by hydrophobic interactions and, in some cases, have hydrophobic alpha-helical regions that interact with the hydrophobic interior of the membranes.