Among the boron trihalides, which is the strongest Lewis acid?

The correct option is C $BB{r}_3$
Lewis acid is an electron pair acceptor which accepts electron pairs to the vacant p or d orbital.
Back bonding occurs due to the lone pair of electrons of halogen atom which increases the electron density on the central atom, which reduces its Lewis acidic character.
This back bonding is more in the case of $B{F}_3$​ due to similar size of orbitals of boron and fluorine atoms, whereas it decreases down the group due to the increasing size of orbitals as we move from $F$ to $Br$.

Hence, $BB{r}_3$ has the highest acidic character and is the strongest lewis acid.