The actinoid contraction is more pronounced than the lanthanoid contraction primarily because of the differences in electron shielding and effective nuclear charge. The correct answer is (B) "5f orbitals are more diffused as compared to 4f orbitals."
Here's an explanation:
(A) Energy of 4f and 5f orbitals being the same is not a significant factor in the actinoid contraction. The primary reason for the actinoid contraction is the poor shielding of the nuclear charge by inner electrons.
(B) The 5f orbitals are indeed more diffused compared to the 4f orbitals. This means that the 5f electrons are further away from the nucleus and are less effective at shielding the outer electrons from the increasing nuclear charge as you move across the actinoid series. This leads to a stronger pull from the nucleus on the outermost electrons, causing a greater contraction in atomic size.
(C) While it's true that actinoids show a greater range of oxidation states than lanthanides, this is not the primary reason for the actinoid contraction.
(D) The reactivity of actinides is not directly related to the actinoid contraction. The actinoid contraction is more about the decreasing atomic size and increasing effective nuclear charge as you move across the actinoid series, which affects various chemical properties but doesn't necessarily make them highly reactive.
So, option (B) is the correct explanation for the greater actinoid contraction compared to lanthanoid contraction.
