Hydrometer is based on the principle of
Archimede's principle
Hydrometer:
Principle of superposition:
When two or more waves overlap in space, the resultant disturbance equals the algebraic sum of the individual disturbances, according to the superposition principle.
Principle of momentum:
If two objects collide, the total momentum before and after the collision will be equal if no external force acts on the system, according to the principle of conservation of momentum.
Hamilton's principle:
The motion of a dynamical system in a given time interval is such that the action integral is maximized or minimized. (The action integral is almost always minimized in practice.) This is known as Hamilton's principle.
Hence, Option(B) is correct.