The correct option is A BF3
There is a stronger 2pπ−2pπ back bonding in BF3 (lone pair of F donated into vacant orbital of B) and weakest in BI3 (2pπ—5pπ). This is due to the large size of 5p orbital which results in lesser overlapping. As the size of the orbital increases, the extent of back bonding decreases. Hence, BF3 has the strongest partial double bond character.
Extent of back Bonding order is: