The correct option is C. Osmosis. Detailed Explanation:

1. Definition of Osmosis:
2. Osmosis is the process by which water molecules move across a selectively permeable membrane from an area of lower solute concentration (higher water concentration) to an area of higher solute concentration (lower water concentration). This movement continues until there is an equal concentration of solute on both sides of the membrane.
3. Selectively Permeable Membrane:
4. A selectively permeable membrane allows certain molecules or ions to pass through it while blocking others. In the case of osmosis, the membrane is permeable to water but not to solutes (like salt or sugar). This characteristic is crucial for the process of osmosis.
5. Concentration Gradient:
6. The concentration gradient refers to the difference in concentration of solutes between two areas. In osmosis, water moves down its concentration gradient, meaning it moves from an area where water is more abundant (and solute is less concentrated) to an area where water is less abundant (and solute is more concentrated).
7. Driving Force:
8. The driving force behind osmosis is the tendency of systems to reach equilibrium. Water moves to balance the solute concentrations on both sides of the membrane, which is a fundamental principle of biological systems.

Why Other Options Are Incorrect:

• A. Active Transport:
• Active transport is the movement of molecules across a membrane against their concentration gradient, which requires energy (usually in the form of ATP). Since osmosis involves the movement of water down its concentration gradient without the use of energy, this option is incorrect.
• B. Diffusion:
• Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration. While osmosis is a type of diffusion (specifically of water), diffusion does not specifically refer to water movement across a membrane. Therefore, while related, diffusion is not the correct term for the process described in the question.
• D. Facilitated Diffusion:
• Facilitated diffusion is a type of passive transport that involves the movement of molecules across a membrane via specific transport proteins. This process also does not require energy and occurs down the concentration gradient. However, it specifically refers to solutes (like glucose or ions) and not water. Thus, it does not accurately describe the movement of water across a membrane.

Summary of Key Points:

• Osmosis is the movement of water across a selectively permeable membrane driven by a concentration gradient.
• It occurs from an area of lower solute concentration to an area of higher solute concentration.
• Osmosis is a passive process and does not require energy.
• It is distinct from active transport, diffusion, and facilitated diffusion, which involve different mechanisms and substances.

This understanding of osmosis is crucial for grasping how cells maintain homeostasis and manage water balance, which is vital for their survival and function.