${\mathrm{N}}_2$ and ${\mathrm{O}}_2$ are converted to monopositive cations ${\mathrm{N}}_2^{+}$ and ${\mathrm{O}}_2^{+}$ respectively. Which is incorrect?
(According to Molecular orbital theory)

Decreasing order of stability of ${\mathrm{O}}_2,\mathrm{ }{\mathrm{O}}_2^{-},\mathrm{ }{\mathrm{O}}_2^{+}$ and ${\mathrm{O}}_2^{2-}$ is:

Among the following molecules/ ions, 

${\mathrm{C}}_2^{2-},\mathrm{ }{\mathrm{N}}_2^{2-},{\mathrm{O}}_2^{2-},\mathrm{ }{\mathrm{O}}_2$

Which one is diamagnetic and has the shortest bond length?