Both simple diffusion and facilitated diffusion
A) Transport molecules only along the concentration gradient and do not use energy
B) Transport molecules only along the concentration gradient by the use of energy
C) Transport molecules against the concentration gradient and do not use energy
D) Transport molecules against the concentration gradient by the use of energy

Hint: Diffusion is the process which involves transport of molecules from a region of higher concentration to a region of lower concentration. The liquid and gaseous particles are often loosely arranged and thus they have a tendency of rapid movement due to which their diffusion takes place.

Complete answer:
The molecules can be transported via two means namely the active transport and the passive transport. In active transport molecules are transported across the membrane against the concentration gradient with the help of energy while passive transport involves movement of molecules along the concentration gradient therefore no energy is required. Both the mentioned type of diffusion that is the simple diffusion and the facilitated diffusion are carried out via passive mode of transport means they transport molecules along the concentration gradient without employing any energy.
However simple diffusion involves the transport of molecules down the concentration gradient between the phospholipid layers while facilitated diffusion involves the use of specialized membrane channels or the protein channels present in the plasma membrane or cell membrane to transport the substances.

So the correct answer is option “A” that is both simple diffusion and facilitated diffusion transports molecules only along the concentration gradient and do not use energy.

Note: Simple diffusion aids in the entry and exit of carbon dioxide, oxygen and the majority of lipids. However, a polar and charged molecule enters and exits the cell by means of facilitated diffusion. Both these diffusions employ the movement of molecules down the concentration gradient.