The correct answer is A. Active transport.
Active transport is the movement of molecules across a cell membrane against a concentration gradient, using energy from ATP. This is in contrast to passive transport, which is the movement of molecules down a concentration gradient, without the use of energy.
Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. This process does not require energy, and it will continue until the concentration of the molecules is equal in both areas.
Osmosis is the movement of water across a semipermeable membrane. Water will always move from an area of high water concentration to an area of low water concentration. If a cell is placed in a solution that has a higher concentration of solutes than the cell, water will move out of the cell, causing the cell to shrink. If a cell is placed in a solution that has a lower concentration of solutes than the cell, water will move into the cell, causing the cell to swell.
In conclusion, the movement of molecules against a concentration gradient is called active transport. This process requires energy from ATP, and it is in contrast to passive transport, which is the movement of molecules down a concentration gradient, without the use of energy.