Which ionisation potential $(IP)$ in the following equations involves the greatest amount of energy?

• A
  $Na \to Na^{+} + e^-$
• B
  $K^{+} \to K^{2+} + e^-$
• C
  $C^{2+} \to C^{3+} + e^-$
• D
  $Ca^{+} \to Ca^{2+} + e^-$