In the process of osmosis
A.Both protoplasm and cell wall act as a single layer
B.Only protoplast acts as a single layer
C.Only cell membrane acts as a single layer
D.None of the above

Hint: The plasma membrane only acts as an ion exchanger which allows passing of materials. This is semi-permeable and thus maintains the concentrations across membranes.

Complete step by step answer:
-Osmosis is the movement of solvent from high concentration to low concentration while solutes move from low concentration of solvent to high concentration. It occurs when there is a travel of substances to maintain concentrations. The ion channels help in active transport of solutes from the cell to the external environment or from external environment to cell.
-There are three types of solutions where the osmoregulation is concerned. Isotonic solutions are solutions where there is equal concentration of solutes and no movement. Hypertonic solutions are solutions with higher concentrations of solute. Hypotonic solutions are solutions with lesser concentrations of solute.
-The plasma membrane is semi-permeable and allows certain materials only to pass. The ions which maintain the concentration are easily passing through the membranes. Thus, the cell membrane only acts as the single layer in the osmosis.

So, the correct option is C.

Note:
Osmosis is different from diffusion as it is not in a gaseous state. Osmosis is a type of diffusion and not every diffusion is osmosis.