Final answer:
Sodium (A) has the smallest first ionization energy among the given options.
Step-by-step explanation:
The element with the smallest first ionization energy is Sodium (A).
Ionization energy is the amount of energy required to remove an electron from an atom or ion.
In general, ionization energy tends to decrease down a group and increase across a period in the periodic table.
Sodium, being at the bottom left of the periodic table, has the lowest first ionization energy among the given options.