What is the definition of ionization energy for an atom?

• A
  The energy required to remove an electron from the outermost shell of an isolated gaseous atom in its ground state.
• B
  The energy released when an electron is added to an isolated gaseous atom.
• C
  The energy required to excite an electron from the ground state to an excited state.
• D
  The energy change when an atom gains a proton.