The tendency of $3d-$ metal ions to form stable complexes is due to their

Assertion : Cuprous ion $(Cu^+)$ has unpaired electrons while cupric ion $(Cu^{2+})$ does not.
Reason : Cuprous ion $(Cu^+)$ is colourless whereas cupric ion $(Cu^{2+})$ is blue in the aqueous solution.

The $3d-$ elements show variable oxidation states. What is the maximum oxidation state shown by the element $Mn$?

Although Zirconium $(Zr)$ belongs to the $4d$-transition series and Hafnium $(Hf)$ belongs to the $5d$-transition series, they show similar physical and chemical properties because . . . . . . .

Match the following elements in List-$I$ with their respective groups in List-$II$.

List-$I$ (Elements)	List-$II$ (Group)
$A$. $Mn, Tc, Re$	$IV$. $7$
$B$. $Zn, Cd, Hg$	$I$. $12$
$C$. $Ti, Zr, Hf$	$II$. $4$
$D$. $Ga, In, Tl$	$V$. $13$

The correct answer is:

Why do the transition elements exhibit variable oxidation states?