Which pair of elements from the following has half-filled $d$-orbitals in their observed electronic configuration?

Which of the following has the most unpaired $d$-electrons?

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:

The electronic configuration of chromium is

Which of the following metal ions has a calculated magnetic moment value of $\sqrt{24} \ BM$?

What are $f$-block elements? Give their characteristics.