Final answer:
The plasma membrane is the part of the cell that encloses the cytoplasm and forms the cell's outer boundary, providing protection, support, and selective permeability.
Step-by-step explanation:
The part of a cell that encloses the cytoplasm and forms the outer boundary of the cell is known as the plasma membrane or cell membrane. This membrane is made up of a phospholipid bilayer with embedded proteins and cholesterol, which separates the internal contents of the cell from its surrounding environment. The plasma membrane serves multiple functions: it acts as a barrier, regulates the passage of substances into and out of the cell, protects the cell, and supports cell structure. It exhibits selective permeability, allowing some substances to pass through while restricting others, thus maintaining the necessary conditions for the cell's survival.