Which of the following is a type of passive transport?

Diffusion is a fundamental type of passive transport that occurs when molecules move from an area of higher concentration to an area of lower concentration. This movement happens naturally, driven by the concentration gradient, without the need for cellular energy (ATP). In many biological contexts, diffusion is crucial for processes such as gas exchange in the lungs and the movement of nutrients and waste across cell membranes.

In contrast, the other options presented involve mechanisms that either require energy or do not fit the definition of passive transport. Pinocytosis and endocytosis refer to forms of active transport where the cell engulfs material, and these processes require energy to facilitate the movement of larger molecules or particles across the cell membrane. Active transport, as the name suggests, encompasses transport processes that require cellular energy to move substances against their concentration gradient. Thus, diffusion stands out as the correct answer due to its characteristic reliance on natural concentration gradients rather than energy expenditure.