Which of the following processes includes all the others?

a. osmosis

b. diffusion of a solute across a membrane

c. passive transport

d. transport of an ion down its electrochemical gradient

Passive transport.

Explanation:

The transport of molecules across the plasma membrane can occur by the two process - active transport and passive transport. No amount of energy is required during the active transport.

Passive transport includes the diffusion and osmosis. Both the diffusion and osmosis do not require energy and the molecules move down from the region of higher concentration towards the region of lower concentration. Small molecules, ions and proteins can go through the passive transport.

Thus, the correct answer is option (c).