Explanation:

- Archimedes Principle (Option 1):

- Widely used to measure the buoyancy of fluids.

- Hydrometers and lactometers work based on this principle by floating in liquids and providing readings based on liquid displacement.

- Correct Answer: Hydrometers and lactometers are based on Archimedes' principle.

- Special Relativity (Option 2):

- Theory by Albert Einstein.

- Deals with the physics of moving bodies at high speeds, nearing the speed of light.

- Not related to measuring liquid density.

- Relativity (Option 3):

- Often refers to Einstein's broader theories, including both special and general relativity.

- Focuses on gravity and the fabric of spacetime.

- Not connected to fluid measurements.

- Uncertainty (Option 4):

- Refers to the Heisenberg Uncertainty Principle in quantum mechanics.

- Relates to the precision limits of measuring particles.

- Not applicable to fluid density or purity measurement.