The valency of an element refers to the ability of an atom to bond with other atoms and form chemical compounds. It is determined by the number of electrons in the outermost shell of an atom, also known as the valence shell.

Option 1, which states that the valency depends on the total number of protons in an atom, is incorrect. The number of protons in the nucleus of an atom determines the element`s atomic number, which defines its identity, but it does not directly influence the valency.

Option 2, which suggests that the valency depends on the mass number of an atom, is also incorrect. The mass number represents the total number of protons and neutrons in an atom`s nucleus and does not affect the valency.

Option 3, stating that the valency depends on the total number of neutrons in an atom, is incorrect as well. Neutrons are neutral particles and do not participate in the formation of chemical bonds.

Option 4, the correct answer, states that the valency depends on the total number of electrons in the outermost shell of an atom. The outermost shell is also referred to as the valence shell, and the electrons in this shell are the ones involved in bonding with other atoms