Molecules in which bond angle increases due to back bonding are:

Molecules is bond angle increases due to back bonding are $\mathrm{O}{\left({\mathrm{SiH}}_3\right)}_2$ and $\mathrm{N}{\left({\mathrm{SiH}}_3\right)}_3$.

Although it is unclear, based on what I understand, I believe have more bond

angle because of presence of only one lone pair while on $\mathrm{O}{\left({\mathrm{Sih}}_3\right)}_2$ two lone pair are present so due to repulsion $\left({\mathrm{SIH}}_3\right)$come nearer to each other and bond angle get decrease on $\mathrm{O}{\left({\mathrm{SiH}}_3\right)}_2$.

So, $\mathrm{N}{\left({\mathrm{SiH}}_3\right)}_3$ have more bond angle.

Hence option B and C is correct.