Which of the following represents an ion? a 4he b na 2+ c ca d mg

Na2+(answer  B)  represent ions.

This is because it has a charge of 2+. ion is formed when an  atom gain or lose  electrons.  when an atom lose electrons it form ion which is positively  charged . On the other hand when atom gain electrons it forms ion of negatively charged. Na2+  has a charge of 2+ since it loses two electrons.

if two electrons have the same n, the same ℓ, and the same ml, then they are in the same orbital. the pauli exclusion principle states that no two identical fermions (a fancy word for electrons and other subatomic particles like electrons) may occupy the same quantum state in an atom simultaneously.