The correct option is B. Osmosis. Detailed Explanation:

1. Understanding the Terms:
2. Selectively Permeable Membrane: This is a type of biological membrane that allows certain molecules or ions to pass through it while blocking others. This property is crucial for maintaining the internal environment of cells.
3. Concentration Gradient: This refers to the difference in the concentration of a substance (in this case, water) across a space or membrane. Water will naturally move from an area of higher concentration to an area of lower concentration.
4. What is Osmosis?:
5. Osmosis is the specific process by which water molecules move across a selectively permeable membrane. It occurs when there is a difference in water concentration on either side of the membrane. Water will move from the side where it is more concentrated (or where there are fewer solute particles) to the side where it is less concentrated (or where there are more solute particles) until equilibrium is reached.
6. Why is Osmosis the Correct Answer?:
7. Osmosis is driven by the concentration gradient of water. It is a passive process, meaning it does not require energy (ATP) to occur. The movement of water continues until the concentrations on both sides of the membrane are equal, achieving a state of equilibrium.

Why the Other Options are Incorrect:

• A. Active Transport:
• Active transport is the movement of substances across a membrane against their concentration gradient, which requires energy (usually in the form of ATP). Since osmosis involves movement along the concentration gradient (from high to low concentration), this option does not apply.
• C. Diffusion:
• Diffusion is the process by which molecules spread from an area of high concentration to an area of low concentration. While water can diffuse, the term "diffusion" does not specifically refer to the movement of water across a membrane. Therefore, while osmosis is a type of diffusion (specifically for water), the term "diffusion" alone does not capture the selective permeability aspect of the membrane.
• D. Endocytosis:
• Endocytosis is a process by which cells engulf substances from their external environment, bringing them into the cell in vesicles. This process is not related to the movement of water across a membrane due to a concentration gradient. It is an active process that requires energy and is used for larger molecules or particles, not for the passive movement of water.

Summary of Key Points:

• Osmosis is the movement of water across a selectively permeable membrane due to a concentration gradient.
• It is a passive process that does not require energy.
• Water moves from an area of higher concentration to an area of lower concentration until equilibrium is reached.
• Other processes like active transport, diffusion, and endocytosis do not accurately describe the specific movement of water in this context.

This understanding of osmosis is fundamental in biology, especially in topics related to cell biology, physiology, and the movement of substances in and out of cells.