Assertion (A) : $B{F}_3$ is a weaker Lewis acid than $BC{l}_3$.
Reason (R) : $B{F}_3$ is is less electron-deficient than $BC{l}_3$.

The correct option is A (A) is correct; (R) is correct and (R) correctly explains (A).
$B{F}_3$ is a weaker Lewis than $BC{l}_3$ because of the more effective back-bonding in boron trifluoride. $B$ and $F$ belong to the same period and this the back-bonding (donation of a lone pair on $F$ into the pure, unhybridized $2p$ orbital of $B$. As for $BC{l}_3$, the back-bonding is not as effective because of the difference in energies of the pure $2p$ orbital of $B$ and the orbital occupied by the lone pairs of $Cl$. Thus, $B{F}_3$ is a weaker Lewis acid.

Thus, both (A) and (R) are correct and the latter accurately explains why (A) is true!