Using $MO$ theory, predict which of the following species has the shortest bond length ?

Correct decreasing order of bond order of oxygen species in $I$ to $IV$

$(I)$   $Na_2O_2$

$(II)$  $O_2 [AsF_6]$

$(III)$  $CsO_2$

$(IV)$  $O_2$

Consider the species $CH _{4}, NH _{4}^{+}$and $BH _{4}^{-}$. Choose the correct option with respect to the there species …. .

Which of the following species exhibits the diamagnetic behaviour ?

Sum of bond order of $\mathrm{CO}$ and $\mathrm{NO}^{+}$is____________.