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