Passage of water across a selectively permeable membrane is

The correct option is osmosis.

Step-by-step Explanation:

1. Understanding the Key Terms:
- Selectively permeable membrane: A barrier (like a cell membrane) that allows only certain substances (such as water and small solutes) to pass through while restricting others.
- Solvent vs. Solute: In biological systems, water is the universal solvent, whereas dissolved substances (like salts or sugars) are solutes.

2. Analyzing the Options:
- Osmosis: This is specifically defined as the net movement or diffusion of water (solvent) molecules across a selectively permeable membrane. Water moves from a region of higher water concentration (low solute concentration) to a region of lower water concentration (high solute concentration) without requiring cellular energy (passive transport). Since the question asks about the passage of water across a selectively permeable membrane, this matches perfectly.
- Active transport: This process moves solute molecules or ions across a cell membrane against their concentration gradient (from low to high concentration), which requires energy in the form of ATP.
- Facilitated diffusion: This is a passive transport process where specific solute molecules (like glucose or ions) pass across the membrane with the help of specialized transmembrane proteins (carrier or channel proteins).
- Pinocytosis: Often referred to as "cell drinking," this is a form of active endocytosis where the cell engulfs droplets of extracellular fluid in small vesicles. It is a bulk transport mechanism rather than the movement of water molecules directly across a membrane.

3. Conclusion:
Therefore, the passive diffusion of water molecules specifically across a selectively permeable membrane is called osmosis.