The bond dissociation energy of B – F in BF3 is 646 kJ mol–1. where as that of C – F in CF4 is 515 kJ mol–1. The correct reason for higher B – F bond dissociation energy as compared to that of C – F bond is

In BF3 due to back π-bonding the bond order is more and hence bond dissociation energy is more [646KJ/mole]Where as in case of CF4 there is no such back π-bond formation, hence the C - F bond order is less so bond dissociation energy for C - F bond is low [515KJ/mole]