If other factors remain constant,which of the following bond orders corresponds to the shortest bond length?

The bond order of a molecule is given by:

As per molecular orbital theory,the pair of molecules which do not exist is:

Which of the following is diamagnetic?

Consider the following changes $I$ and $II$ as shown in the figure. The correct statements about these changes $(I)$ and $(II)$ in accordance with $MO$ theory are:
$A$) In $(I)$ bond order increases by $0.5$ from the existing value.
$B$) In $(II)$ bond order decreases by $0.5$ from the existing value.
$C$) In both $(I)$ and $(II)$ magnetic property is changed.
$D$) In both $(I)$ and $(II)$ magnetic property is not changed.

The diamagnetic species among the following is $.....$