( A ) stronger $\sigma$ bond between B and F in $BF_3$ as compared to that between C and F in $CF_4$
( B ) smaller size of B-atom as compared to that of C-atom
( C ) lower degree of $p \pi - p \pi$ interaction between B and F in $BF_3$ than that between C and F in $CF_4$.
( D ) significant $p \pi - p \pi$ interaction between B and F in $BF_3$ whereas there is no possibility of such interaction between C and F in $CF_4$