@tech-with-us A species which is more electron deficient will electron easily and hence will be more acidic.this trend is due to pie back bonding. All halides have lone pairs, they donate one lone pair to boron by overlapping with the p orbital.
We see that boron is present in all compounds of the given option.... So we can check with halides having maximum bond length... Since the size of Br>Cl>F, hence due to large radii the B-Br will have maximum bond length then B-Cl and least acidic in the give pair will be B-F
..... Else use @mayank-sewatkar Method