Why do the transition elements exhibit variable oxidation states?

(N/A) Transition elements exhibit variable oxidation states because their $(n-1)d$ and $ns$ orbitals have comparable energies.
The general electronic configuration of transition elements is $(n-1)d^{1-10}ns^{1-2}$.
Since the energy difference between the $(n-1)d$ and $ns$ orbitals is very small,electrons from both shells can participate in chemical bond formation.
For the first transition series,the $4s$ electrons are primarily responsible for lower oxidation states,while both $3d$ and $4s$ electrons contribute to higher oxidation states.
The table below illustrates the oxidation states of the first transition series:

$Sc$	$Ti$	$V$	$Cr$	$Mn$	$Fe$	$Co$	$Ni$	$Cu$	$Zn$
	$+2$	$+2$	$+2$	$+2$	$+2$	$+2$	$+2$	$+1$	$+2$
$+3$	$+3$	$+3$	$+3$	$+3$	$+3$	$+3$	$+3$	$+2$
	$+4$	$+4$	$+4$	$+4$	$+4$	$+4$	$+4$
		$+5$	$+5$	$+5$
			$+6$	$+6$	$+6$
				$+7$