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# Which of the following is the correct order for increasing the Lewis acid character?

$\begin{array}{1 1}(a)\;BF_3 < BBr_3 < BCl_3\\(b)\;BF_3 < BCl_3 < BBr_3\\(c)\;BCl_3 < BBr_3 < BF_3\\(d)\;BBr_3 < BCl_3 < BF_3\end{array}$

Lewis acidic nature increases from $BF_3$ to $BBr_3$ .
The effectiveness of formation of $\pi$-back bonding between halogen and boron atom decreases from B-F to B-Cl to B-Br.
Smaller fluorine atom forms $\pi$-bond effectively with smaller boron atom.
Hence (b) is the correct answer.