Final answer:
Fluid moves up an osmometer due to osmotic pressure, which is created when water moves across a semipermeable membrane to balance solute concentrations, causing the fluid level to rise until equilibrium is reached.
Step-by-step explanation:
The reason fluid moves up the osmometer is due to osmotic pressure. Osmotic pressure is the pressure created when a solvent, such as water, moves across a semipermeable membrane separating two solutions with different solute concentrations. In this process, water moves from an area of lower solute concentration (and thus higher water concentration) to an area of higher solute concentration (lower water concentration) to try to equalize solute concentration on both sides of the membrane.
This movement of water continues until the osmotic pressure, which is proportional to the height of the fluid column, balances out the tendency for water to move across the membrane. This causes the fluid level in the osmometer to rise, which is often seen in the case where an osmometer tube is placed in a solution of higher solute concentration than the solution inside the osmometer.
Thus, osmotic pressure, not gravity, centrifugal force, or capillary action, is responsible for the upward movement of fluid in an osmometer.