In which of the $f$-block elements is the number of electrons in the $(n-2)f$ subshell zero?

Most transition metals exhibit several characteristic properties. Which of the following statements are correct?
$I$: Form sets of compounds which display different oxidation states of the metal.
$II$: Form coloured ions in solution.
$III$: Burn vigorously in oxygen.
$IV$: Act as catalysts.

Given below are two statements :
Statement $I$ : The higher oxidation states are more stable down the group among transition elements unlike $p-$block elements.
Statement $II$ : Copper can not liberate hydrogen from weak acids.
In the light of the above statements,choose the correct answer from the options given below :

The metal with the highest oxidation state present in $K_2CrO_4$,$NbCl_5$,and $MnO_2$ is

The electronic configuration of ${ }_{59} Pr$ $(Praseodymium)$ is

The $3d$ elements show variable oxidation states because the energies of the following sets of orbitals are almost similar: