Number of colourless lanthanoid ions among the following is . . . . .
$Eu^{3+}, Lu^{3+}, Nd^{3+}, La^{3+}, Sm^{3+}$

Given below are two statements:
Statement $I$: In the Lanthanoids,the formation of $Ce^{+4}$ is favoured by its noble gas configuration.
Statement $II$: $Ce^{+4}$ is a strong oxidant reverting to the common $+3$ state.
In the light of the above statements,choose the most appropriate answer from the options given below:

What is the correct order of ionic radii for the $+3$ oxidation state in $Ce, La, Pm,$ and $Yb$?

What are alloys? Name an important alloy which contains some of the lanthanoid metals. Mention its uses.

The lanthanide contraction is responsible for the fact that

If $Ln$ represents a lanthanoid element,find the general formula of lanthanoid hydroxide.