What structure protects the cell by acting as a barrier?

The cell membrane is the correct answer because it serves as a protective barrier that regulates the movement of substances in and out of the cell. Composed mainly of a phospholipid bilayer with embedded proteins, the cell membrane maintains the cell's integrity and environment by controlling the entry and exit of ions, nutrients, and waste products. This selective permeability is crucial for maintaining the internal balance of the cell, known as homeostasis.

In contrast, the nucleus is the control center of the cell that houses genetic material but does not act as a barrier. The cytoplasm is the jelly-like substance within the cell where cellular processes occur but does not serve a protective role like the membrane does. Chromosomes, which are structures made of DNA, contain genetic information but are not involved in protecting the cell's outer structure.