Solution

In $$\\ { BF }_{ 3 }\\ $$ due to back bonding means electron donation by fluorine to boron electron density increases in boron and hence its lewis acid character decreases.
As we move from fluorine to chlorine size of orbitals increases and hence there isn't much effective back bonding. Similarly $$\\ { BBr }_{ 3 }\\ $$ will have less back bonding due to much greater increases in size of overlapping orbitals.