Which of the following pairs do not constitute resonance structures ?

Explanation

In resonance the position of atoms or nuclei remains fixed, so pairs in (b) constiute resonance. In other pairs, the positions of atoms are changing, therefore (a),(c ), and (d) pairs do not constitute resonance structures
(a) They constitute functional isomerism(nitro to nitrite)
(b) They constitute resonance structures
(c ) They constitute tautomerism (keto and enol) and also show functional isomerism
(d) They constitute positional isomerism (position of double bond has changed) .