Final answer:
Aqueous solutions of La(OH)3, Lu(OH)3, and Ce(OH)3 are colorless because they contain colorless ions which do not absorb visible light due to having full or nearly full f-orbitals.
Step-by-step explanation:
Aqueous solutions of La(OH)3, Lu(OH)3, and Ce(OH)3 are colorless because they contain colorless ions. The metal ions in these hydroxides do not have unpaired electrons in their d or f orbitals that might absorb visible light, which is commonly the cause of color in many transition metal complexes. This is due, in part, to the lanthanides having full or nearly full f-orbitals at these oxidation states. Since they do not absorb light in the visible spectrum, the solutions appear colorless to the human eye.
Moreover, the hydroxide ions (OH)- are indeed present in these solutions, as they are part of the hydroxide compounds in question. Additionally, their colors are not related to concentration; even if these solutions were not highly diluted, they would still appear colorless because color is a property of the ions themselves and not their concentration. Therefore, options b) strong color masking, c) the hydroxide ions are not present, and d) they are highly diluted solutions are incorrect explanations for the colorless nature of these solutions.