The movement of fluid up or down a narrow tube is

Step 1: Analyze the question The question asks for the name of the process in which a fluid rises or falls inside a narrow tube (a capillary tube).

Step 2: Explain the concept When a narrow tube is dipped into a liquid, the liquid rises or falls inside it. This happens because of two forces:

• Adhesion is the attraction between the liquid molecules and the walls of the tube.
• Cohesion is the attraction between the liquid molecules themselves.

If adhesion is stronger than cohesion (e.g. water in glass), the liquid rises and the surface curves upward (concave meniscus). If cohesion is stronger (e.g. mercury in glass), the liquid is depressed and the surface curves downward (convex meniscus). This behavior is called capillarity (or capillary action).

Step 3: Check each option

• A. Osmosis: movement of solvent molecules through a semi-permeable membrane from a dilute to a concentrated solution. It does not involve a narrow tube.
• B. Brownian motion: random zigzag movement of small particles suspended in a fluid, caused by molecular bombardment.
• C. Capillarity: rise or fall of a liquid in a narrow tube due to surface tension, adhesion and cohesion. This matches the question.
• D. Diffusion: spreading of particles from a region of high concentration to one of low concentration.

Step 4: Answer The correct option is C. capillarity.

Exam tip: The word "capillary" means "hair-like", so think of a very thin tube. Whenever you see "narrow tube" together with "rise or fall of liquid", the answer is capillarity. Real-life examples include kerosene rising up a lamp wick and water moving up plant stems.