Why Is the Ionization Energy of Potassium (K) Less Than

Multiple Choice

Why is the ionization energy of potassium (K) less than the ionization energy of sodium (Na) ?

A) The ionized electron in K is in a higher energy shell than the ionized electron in Na.
B) Because the core charge remains the same going down a group
C) Because K has more protons in its nucleus than Na.
D) Because the core charge decreases down a group
E) Because the ionized electron in K is in a higher energy shell AND because the core charge of K and Na are the same.