Dear studentThe answer is third part. In this the vacant orbital of boron cannot participitate in bonding due to Back Bonding.Backbonding is a type of resonance.In compounds like BF3, the boron atom has an incomplete octet. The fluorine atom on its side has a lone pair which it can donate to boron. But, flurorine is also a very electronegative element. So, it also has a tendency to take back the electrons that it had donated to boron. This way, the lone pair of electrons keepjumping between fluorine and boron.This is called back bonding. This provides thelone pair of electrons more number of exchange positions (which simply means more space). As a result, the molecule becomes more stable. RegardsArun (askIITians forum expert)
