Which elements of the $3d$ series do not show variable valency?

Observe the following statements:
$a$) The basic strength of $Lu(OH)_3$,$Gd(OH)_3$ and $Ce(OH)_3$ follows the order: $Ce(OH)_3 > Gd(OH)_3 > Lu(OH)_3$
$b$) $O^{2-}$,$N^{3-}$,$F^{-}$ and $Na^{+}$ are isoelectronic species
$c$) The sizes of $Zr$ and $Hf$ are approximately the same
The correct statements are:

$Zn$ and $Hg$ do not show variable valency like other $d$-block elements because ...

Find the value of spin-only magnetic moment for chromium cation in $+2$ state. (in $BM$)

Among the following options,identify the element which exhibits the greatest number of oxidation states.

Which of the following statements are correct? $(1)$ Manganese can exhibit a $+7$ oxidation state. $(2)$ Zinc forms colored ions. $(3)$ $[CoF_6]^{3-}$ is diamagnetic. $(4)$ $Sc$ can exhibit a $+4$ oxidation state. $(5)$ $Zn$ exhibits only a $+2$ oxidation state.