${BX}_3+{NH}_3\to {BX}_3.{NH}_3+$ Heat of adduct formation ($\Delta H$)
The numerical value of $\Delta H$ is found to be maximum for:

The correct option is A ${BF}_3$
${BX}_3.{NH}_3$

Adduct ${form}^n$ order ${BI}_3>BB{r}_3>{BCl}_3>B{F}_3$

As in ${BF}_3$ strong back bonding occur. So adduct ${form}^n$ decreases. Back bonding $\alpha \frac{1}{adduct{form}^n}$