Which of the below molecule(s) will have an odd-numbered molecular ion?

Thumb rule for knowing either the molecular ion is odd or even is to know that either the molecule contain odd or even number of nitrogen atoms. If a molecule contains odd number of nitrogen atoms the the molecular ion for particular molecule formed after the removal of one electron will be odd too. In this question the molecules having or about to give odd molecular ions are shown below with odd molecular mass (RED).

The answer is 4 i don't know if its right let me see.