Which of the Following Is the Best Definition of First

Which of the following is the best definition of first ionization energy?

A) The energy required to remove one electron from a neutral gaseous atom of an element
B) The energy barrier that must be overcome to start a chemical reaction
C) The energy required to raise an electron in the ground state into the first excited state
D) The energy defined by mv²
E) The energy needed to reduce a species in the aqueous phase to its minimum (first) reduction potential