Which of the following are the applications of Archimedes' Principle?

Archimedes' principle states that when an object is partially or fully submerged in a liquid, it experiences an upward force that is equal to the weight of the liquid displaced by the object.

Archimedes' principle is used in designing submarines and ships. This is because we need to design the ships and submarines in such a way that they will experience enough upthrust in order to float on the water. Submarines are especially difficult to design as they need to be made in such a way that they can float or sink in water as per the command. For this purpose, they have tanks that can be filled with water or emptied so that the overall density of the submarine will be more or less than the seawater.

It is also used in a lactometer to determine the purity of a sample of milk. This is done by checking the density of the sample as the adulteration changes the density of the milk.

It is also used in hydrometers to determine the density of liquids. This is done by suspending a hydrometer in a jar filled with the liquid till it floats freely. The density is found from the amount of liquid displaced by the hydrometer.

Hence option D is the correct answer.