Hydrometer is based on the principle of

The correct option is B

Archimede's principle

Hydrometer:

1. The hydrometer operates on Archimede's principle.
2. It states that the weight of the liquid object displaced by the immersed portion of the hydrometer equals the weight of the 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.