Answer:
c. a transmembrane protein
Step-by-step explanation:
Transmembrane proteins extend through the lipid bilayer of the cell membrane and transport ions, macromolecules, and other substances across the membrane. Ion channels, which provide a regulated pathway for ions to pass through the membrane, are examples of transmembrane proteins. They allow specific ions to move in or out of cells, maintaining proper ion balance and playing important roles in many physiological processes such as nerve impulse transmission and muscle contraction. Option a refers to the composition of the membrane, while options b and d are not directly related to ion channels.