When are $d$-block elements considered as transition elements?

• A
  In the ground state,the $d$-orbital is completely filled with electrons.
• B
  In the ground state or any stable oxidation state,the $d$-orbital is incompletely filled with electrons.
• C
  In all oxidation states,the $d$-orbital is completely filled with electrons.
• D
  In only one oxidation state,the $d$-orbital is completely filled with electrons.