What structure covers a cell and maintains its integrity?

The cell membrane is the correct answer because it serves as the protective barrier that surrounds the cell, maintaining its shape and integrity. This semi-permeable membrane is crucial for regulating the movement of substances in and out of the cell, allowing essential nutrients to enter while keeping harmful substances out. It also plays a key role in communication with other cells and external signals.

The cell membrane is composed of a lipid bilayer with embedded proteins, which contributes to its flexibility and functionality. This structure is vital for the overall health and operation of the cell, ensuring that it can sustain itself and respond to its environment effectively.

Other structures, such as cytoplasm, endoplasmic reticulum, and nucleus, have specific roles within the cell but do not fulfill the primary function of covering and protecting the cell’s boundary. The cytoplasm is the gel-like substance inside the cell where organelles are located and various cellular processes occur. The endoplasmic reticulum is involved in protein and lipid synthesis, while the nucleus houses the cell's genetic material and is responsible for controlling cellular activities. However, none of these structures directly cover and maintain the integrity of the cell like the cell membrane does.