The correct answer is option 2: Equivalent weight.

Valency refers to the combining capacity of an atom or ion, which determines the number of electrons an atom can gain, lose, or share when forming chemical bonds. Equivalent weight, on the other hand, is a measurement of the mass of a substance required to react with or replace a certain amount of another substance in a chemical reaction.

Valency determines the number of chemical bonds an atom can form, and as the number of bonds changes, the chemical reactions and the amount of substance required to react with it also change. This leads to a change in the equivalent weight.

Option 1, atomic weight, refers to the average weight of an atom of an element, and it is constant for each element regardless of the number of chemical bonds it can form. Option 3, molecular weight, refers to the sum of the atomic weights of all the atoms in a molecule, and it is also constant for a particular molecule. Option 4, density, is a physical property that measures the mass of a substance compacted within a given volume and is not directly related to valency.