A monoatomic Ion that has 18 electrons and +2 charge:

A monoatomic Ion that has 18 electrons and +2 charge is isoelectronic with $\mathrm{Ar}$.

No. of $p=18+2=20$

$\mathrm{Ar}$has $18e^{-}$

A certain element produces an ion with 18 electrons and a positive charge. Remember that an atom's atomic number equals the number of protons it contains, and that atoms have the same number of electrons as protons. The atom in question is thus calcium, which has the atomic number 20. (Ca).

Hence, the option D is correct.