Explanation:

When a bucket is immersed fully in a fluid, only then it can experience an upward force that is equal to the weight of the fluid displaced by it: This statement is incorrect. According to Archimedes' principle, an object submerged in a fluid experiences an upward buoyant force equal to the weight of the fluid it displaces. It doesn't have to be fully immersed; even partial immersion leads to a buoyant force.

Lactometers and hydrometers are based on Archimedes' principle: This statement is correct. Lactometers and hydrometers operate on the principle of buoyancy, which is derived from Archimedes' principle.

Lactometers are used to determine the purity of a sample of milk: This statement is correct. Lactometers are devices used to measure the density or specific gravity of milk, which can be an indicator of its purity.

Hydrometers are used for determining the density of liquids: This statement is correct. Hydrometers are instruments used to measure the density of liquids, and they operate on the principle of buoyancy.