1. Concept: Lanthanoid contraction is the gradual decrease in the atomic and ionic radii of lanthanoid elements.2. Explanation: The 'f' orbitals have poor shielding or screening effect. As the nuclear charge increases across the lanthanoids, the poor shielding by 'f' electrons does not effectively counteract the increased attraction of the nucleus for the outer electrons, leading to a contraction in size.3. Answer: A. Negligible screening effect of 'f orbitals
