Lanthanoid contraction is due to increase in which of the following factor?

Effective nuclear charge

The correct answer is: (1) Effective nuclear charge

Lanthanoid contraction is due to an increase in effective nuclear charge as we move across the lanthanoid series. In the lanthanoid series, the number of electrons in the inner 4f subshell increases, but the additional electrons do not effectively shield the outer electrons from the increasing positive charge of the nucleus. As a result, the effective nuclear charge experienced by the outer electrons increases, leading to a contraction in atomic size. This trend causes a decrease in the atomic and ionic radii of successive lanthanoid elements as we move from left to right in the periodic table.