Mesomeric effect does not operate in:

(a) $\mathrm{H}-\stackrel{+}{\mathrm{C}}=\stackrel{..}{\mathrm{O}}:\leftrightarrow \mathrm{H}-\mathrm{C}\equiv \stackrel{+}{\mathrm{O}}:$

(b) $\stackrel{-}{\mathrm{C}}{\mathrm{H}}_2-\stackrel{+}{\mathrm{N}}\equiv \stackrel{..}{\mathrm{N}}\leftrightarrow {\mathrm{CH}}_2=\stackrel{+}{\mathrm{N}}=\stackrel{-}{\mathrm{N}}$

(c) ${\mathrm{CH}}_3-\stackrel{+}{\mathrm{C}}\mathrm{H}-\underset{..}{\overset{..}{\mathrm{O}}}\mathrm{H}\leftrightarrow {\mathrm{CH}}_3-\mathrm{CH}=\underset{..}{\overset{+}{\mathrm{O}}}\mathrm{H}$

In which of the above pairs of resonance contributors, the structure on the right is an important contributor?

The correct order of $\mathrm{C}-\mathrm{Cl}$ bond length in the given molecules is:

I 

II 

III 

(a) $\mathrm{p}-{\mathrm{O}}_2\mathrm{N}-{\mathrm{C}}_6{\mathrm{H}}_4-\stackrel{+}{\mathrm{C}}{\mathrm{H}}_2$

(b) $\mathrm{p}-{\mathrm{CH}}_3\mathrm{O}-{\mathrm{C}}_6{\mathrm{H}}_4-\stackrel{+}{\mathrm{C}}{\mathrm{H}}_2$

(c) $\mathrm{p}-\mathrm{Cl}-{\mathrm{C}}_6{\mathrm{H}}_4-\stackrel{+}{\mathrm{C}}{\mathrm{H}}_2$

(d) ${\mathrm{C}}_6{\mathrm{H}}_5-\stackrel{+}{\mathrm{C}}{\mathrm{H}}_2$

Correct order of stability of above species is:

The correct order of the bond order of a and b is?