The order of back-bonding in boron halides is BF3>BCl3>BBr3>BI3 due to the strength of overlapping which is in order
2p−2p>2p−3p>2p−4p>2p−9p
Hence, stronger is the back-bonding, lesser the availability of empty p-orbital and thus, weaker the tendency to act as a Lewis acid. So, the correct order of Lewis acidic character of boron trihalides is BI3>BBr3>BCl3>BF3‌. ‌