Correct Option: C. Cell membrane Detailed Explanation: The cell membrane, also known as the plasma membrane, is a crucial structure in living cells that plays a vital role in maintaining homeostasis by regulating the movement of substances in and out of the cell. Here’s a step-by-step breakdown of why the cell membrane is the correct answer:

1. Structure of the Cell Membrane:
2. The cell membrane is primarily composed of a phospholipid bilayer, which consists of two layers of phospholipids. Each phospholipid has a hydrophilic (water-attracting) "head" and two hydrophobic (water-repelling) "tails."
3. This unique structure allows the membrane to be selectively permeable, meaning it can control what enters and exits the cell.
4. Function of the Cell Membrane:
5. The cell membrane contains various proteins that serve different functions, including transport proteins that facilitate the movement of specific molecules across the membrane.
6. It also has receptors that allow the cell to communicate with its environment and respond to external signals.
7. Regulation of Substance Movement:
8. The cell membrane regulates the movement of substances through several mechanisms:
   • Passive Transport: This includes diffusion and osmosis, where substances move across the membrane without the use of energy. For example, oxygen and carbon dioxide can diffuse freely through the membrane.
   • Active Transport: This process requires energy (ATP) to move substances against their concentration gradient. For instance, the sodium-potassium pump actively transports sodium ions out of the cell and potassium ions into the cell.
   • Endocytosis and Exocytosis: These processes involve the membrane engulfing substances to bring them into the cell (endocytosis) or packaging substances in vesicles to expel them from the cell (exocytosis).

Why the Other Options Are Incorrect:

• A. Mitochondria:
• Mitochondria are known as the powerhouses of the cell because they generate ATP through cellular respiration. While they are essential for energy production, they do not regulate the movement of substances in and out of the cell. Their primary function is energy metabolism.
• B. Ribosomes:
• Ribosomes are the sites of protein synthesis in the cell. They translate messenger RNA (mRNA) into proteins. While they are crucial for producing proteins, they do not have any role in regulating the movement of substances across the cell membrane.
• D. Nucleus:
• The nucleus is the control center of the cell, housing the cell's genetic material (DNA) and coordinating activities such as growth, metabolism, and reproduction. However, it does not directly regulate the movement of substances in and out of the cell. The nuclear envelope does have pores that allow certain molecules to pass, but this is not its primary function.

Summary of Key Points:

• The cell membrane is primarily responsible for regulating the movement of substances in and out of the cell.
• It is composed of a phospholipid bilayer that is selectively permeable.
• The membrane utilizes passive and active transport mechanisms to control substance movement.
• Other organelles like mitochondria, ribosomes, and the nucleus have distinct functions unrelated to the regulation of substance movement across the cell membrane.

This understanding of the cell membrane's role is fundamental in biology, especially when studying cellular processes and homeostasis.