Which structure regulates cell entry and exit while providing structural support?

The plasma membrane is the correct answer because it serves as the primary barrier that regulates the entry and exit of substances to and from the cell. This selectively permeable membrane controls the movement of ions, nutrients, and waste products, ensuring that essential materials can enter the cell while keeping harmful substances out.

In addition to its regulatory role, the plasma membrane also provides structural support to the cell by maintaining its shape and integrity. It is composed of a phospholipid bilayer embedded with proteins that facilitate communication and transport across the membrane. This dynamic structure allows the cell to adapt to changes in its environment while protecting its internal components.

In contrast, the cell wall, found in plants and some prokaryotes, provides structural support but does not selectively regulate entry and exit as precisely as the plasma membrane. The nuclear envelope encloses the nucleus and regulates the passage of molecules between the nucleus and the cytoplasm but does not encompass the entire cell. The endosome participates in the transport and sorting of materials within the cell rather than regulating what enters or exits the cell itself.