The common balance works on the principle of equality of(A). Forces(B). Moment of forces(C). Masses(D). Masses of pans

The correct answer is: (B) Moment of forces.

Explanation:
A common balance operates based on the principle of the equilibrium of moments. When the balance is used, weights placed in the pans create moments around the fulcrum (pivot point). The moment of a force is given by the product of the force and the perpendicular distance from the fulcrum.

For equilibrium, the sum of the clockwise moments must equal the sum of the anticlockwise moments. Therefore, the balance is adjusted so that the moments of the weights on both sides are equal, allowing the beam to stay horizontal.

It is not based solely on the equality of forces, masses, or the masses of the pans, as these alone do not account for the distances from the fulcrum. Hence, the principle of equality of the moment of forces governs the functioning of a common balance.