Which mechanism requires energy? exocytosis diffusion of ions through a potassium channel diffusion of oxygen into a red blood cell osmosis

The answer is exocytosis.

Exocytosis is the opposite process to endocytosis. In exocytosis, cell transports molecules, such as proteins, out of the cell and this process requires energy.

Diffusion is a process in which molecules move from areas of high concentration to areas of low concentration and it does not require energy. Osmosis is a net movement of solvent molecules from a higher solute concentration to a lower solute concentration. Osmosis is actually a result of diffusion.