Answer:
The correct answer would be the passive transport.
Passive transport refers to the movement of small molecules such as gases, ions etc across the cell membrane (semi-permeable membrane) without the need of energy.
Passive transport can be of various types such as diffusion, osmosis, and facilitated diffusion.
Diffusion refers to the movement of small molecules (e.g. O₂, CO₂ etc) from the region of high concentration to the region of low concentration across the cell membrane i.e. they move down the concentration gradient. The movement stops as soon as the equilibrium is obtained across the membrane.
Osmosis is a term used for the movement of water from the region of high concentration to the region of low concentration across the semi-permeable membrane.
When diffusion takes place with the help of trans-membrane protein or carrier protein, it is termed as facilitated diffusion. It aids the in diffusion of large molecules or charged ions (Na⁺, K⁺ etc).
