Explanation
The ion that is not a molecular ion is b) NH4+.
A molecular ion is formed when a molecule gains or loses an electron, but NH4+ (ammonium ion)
It is not formed by removing an electron from a neutral molecule; it is formed by adding a proton (H+) to ammonia (NH3).
He+ (helium ion) is also not a molecular ion because it consists of only one atom.
Helium is a noble gas and does not readily form covalent bonds with other atoms to create a molecule.