What is the process in which a cell engulfs extracellular material by inwardly folding its plasma membrane?

The process described in the question is known as endocytosis. This is a mechanism by which cells internalize extracellular material by engulfing it through the inward folding of the plasma membrane. When the plasma membrane folds inwards, it forms a pocket that eventually pinches off, creating a vesicle containing the material taken from outside the cell. Endocytosis allows cells to bring in larger particles, molecules, or even other cells.

This process is essential for various cellular functions, including nutrient uptake, regulation of cell surface receptors, and removal of pathogens. It plays a critical role in maintaining cellular homeostasis and communication with the environment.

The other processes listed do not involve the engulfing of materials in this manner. Exocytosis, for example, is the reverse of endocytosis, where materials are expelled from the cell. Diffusion and osmosis describe the movement of substances across membranes but do not involve the formation of vesicles or the active uptake of materials by the cell membrane.