The correct answer is b) it controls the substances entering and leaving the cell.
The cell membrane, also known as the plasma membrane, is a crucial component of all living cells. Its primary function is to serve as a barrier that regulates the passage of substances in and out of the cell. This selective permeability is essential for maintaining the internal environment of the cell, allowing it to carry out necessary functions while protecting it from external harm.
While the cell membrane does provide some structural support and can help maintain the shape of the cell, its most vital role is in controlling the movement of ions, nutrients, and waste products. By doing so, it helps to maintain homeostasis within the cell, ensuring the right balance of substances is achieved for optimal cellular function.