In lanthanides,with increase in atomic number,atomic radius decreases,except for the element $\underline{ X }$. What is $\underline{ X }$?

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

The correct order of ionic radii of $Ce, La, Pm$ and $Yb$ in $+3$ oxidation state is:

Lanthanoid ions with $4f^7$ configuration are :
$A$. $Eu^{2+}$
$B$. $Gd^{3+}$
$C$. $Eu^{3+}$
$D$. $Tb^{3+}$
$E$. $Sm^{2+}$
Choose the correct answer from the options given below :

All those elements belong to $f-$block whose atomic numbers are

Lanthanum $(La)$ is grouped with $f-$block elements because