What is ionization energy?
a. the minimum energy required to remove one electron from an atom.
b. the maximum energy required to remove one electron from an atom.
c. the maximum energy required to move one electron to the valence shell.
d. the minimum energy required to remove one electron from another electron.

D)energy required to remove a valence electron

Explanation:

Ionization energy can be defined as the atom's ability to and the energy required to remove a valence electron. Ionization energy increases up a group and to the right along a period. So helium has the highest first Ionization energy meaning that it is very hard to remove an electron from helium because octet is already complete.