Which of the following is not used as a Lewis acid?

Among the following molecules,the $N-Si$ bond length is shortest in:

In which of the following,the molecules are arranged in the increasing order of their bond angles?

Observe the following species:
$(i)$ $NH_3$
$(ii)$ $AlCl_3$
$(iii)$ $SnCl_4$
$(iv)$ $CO_2$
$(v)$ $Ag^{+}$
$(vi)$ $HSO_4^{-}$
How many of the above species act as Lewis acids?

The number of $S-S$ bonds in sulphur trioxide trimer $(S_3O_9)$ is

Which of the following is the correct increasing order of $O-O$ bond length in $O_2$,$O_3$,and $H_2O_2$?