Answer
They can lose the d electrons without high energy penalty.
Work Step by Step
Unlike main-group metals that tend to exhibit only one oxidation state, the transition metals often exhibit a variety of oxidation states since they can lose the d electrons without high energy penalty.
This can be explained by taking manganese as an example: the highest oxidation state of manganese is \[+7\], which corresponds to loss of all the electrons in the \[4s\] and \[3d\] orbitals, leaving the noble gas electron configuration. It also exhibits \[M{{n}^{2+}}\] by the loss of two electrons present in the \[~4s\] orbital.
