Th choice which represent a pair of resonance structures is O - N =O and O = N - O.
A resonance pair is another mean of drawing the Lewis structure of a compound. Equivalent Lewis structures are called resonance form. Resonance form is used to spot delocalized electrons; this is done by comparing the electron locations in the two resonance form. If a pair appear in one place in one form and the other pair form and place are different, then the pair is delocalized.