Correct Option: B. Osmosis Detailed Explanation: What is Osmosis? Osmosis is the process by which water molecules move across a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration. This movement occurs in order to equalize solute concentrations on both sides of the membrane. The selectively permeable membrane allows water to pass through while restricting the movement of solutes (like salts or sugars). Why is Osmosis the Correct Answer? 1. Concentration Gradient: Osmosis specifically refers to the movement of water in response to a concentration gradient. When there is a difference in solute concentration across a membrane, water will move toward the area with a higher concentration of solutes to dilute it. This is a passive process, meaning it does not require energy (ATP) to occur.

1. Selectively Permeable Membrane: The term "selectively permeable" indicates that the membrane allows certain substances to pass while blocking others. In the case of osmosis, the membrane permits water to move freely, but not the solutes, which is crucial for the process to occur.
2. Passive Process: Osmosis is a passive transport mechanism, meaning it relies on the natural tendency of molecules to move from areas of higher concentration to areas of lower concentration without the input of energy.

Why the Other Options are Incorrect: A. Active Transport - Definition: Active transport is the movement of molecules across a membrane against their concentration gradient, which requires energy (usually in the form of ATP). - Why It's Wrong: Osmosis does not require energy and moves water along the concentration gradient, not against it. Therefore, it does not fit the definition of active transport. C. Facilitated Diffusion - Definition: Facilitated diffusion is the process by which substances move across a membrane with the help of transport proteins, also down their concentration gradient, but it typically refers to solutes rather than water. - Why It's Wrong: While facilitated diffusion is a passive process like osmosis, it specifically involves solutes and not the movement of water. Osmosis is distinct because it specifically describes the movement of water molecules. D. Endocytosis - Definition: Endocytosis is a process by which cells engulf substances from their external environment, bringing them into the cell in vesicles. This process requires energy. - Why It's Wrong: Endocytosis is not related to the movement of water across a membrane. It involves the uptake of larger particles or fluids and is an active process, which is contrary to the passive nature of osmosis. Summary of Key Points:

• Osmosis is the movement of water across a selectively permeable membrane driven by a concentration gradient.
• It occurs from areas of lower solute concentration to areas of higher solute concentration.
• Osmosis is a passive process that does not require energy.
• Other processes like active transport, facilitated diffusion, and endocytosis involve different mechanisms and energy requirements, making them unsuitable answers for this question.

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