The hardness of $Cr$ is due to $......$ and metallic lustre is due to $.......$

The correct statement about iron includes:
$I$) The highest oxidation state of iron is $+ 6$ in $K_2FeO_4$.
$II$) The iron shows $+ 2$ oxidation state with six electrons in the $3d$ orbitals.
$III$) The common oxidation state of iron is $+ 3$ with five unpaired electrons in the $3d$ orbital.

The metal ion (in gaseous state) with the lowest spin-only magnetic moment value is .... .

All transition elements are $d$-block elements,but all $d$-block elements are not transition elements. Explain.

Which of the following represents the decreasing order of the second ionization enthalpy $(I.P_2)$?

Identify the correct decreasing order of densities of $d-$block elements.