Diffusion is a process by which:

Correct option is A

Diffusion is a process by which oxygen can move across the plasma membrane.
 Diffusion is the passive movement of molecules from an area of higher concentration to an area of lower concentration, and it plays a crucial role in the exchange of gases, such as oxygen and carbon dioxide, across the cell membrane in organisms.
 This process allows oxygen to move from areas of higher concentration, such as the environment or bloodstream, to areas of lower concentration, such as cells that need oxygen for respiration.