Which of the following substances will move across the plasma membrane?

Iodine would move through the membrane via diffusion, water through osmosis. The cell membrane is selectively permeable, so macromolecules like starch and sugar can not move through via diffusion or osmosis. They're way too big. They could only cross through the membrane via active transport, or pinocytosis (cell eating). Diffusion and osmosis are both pive transport, not requiring energy from the cell. Active transport does require the cell's energy.