Explanation

(a)
Boron trihalides are Lewis acid. The order of their acidic strength is as
$\ \ \ \ \ \ \ \ \ \ BF_{3} < BCl_{3} < BBr_{3} < BI_{3}$
In the boron halides, a $p\pi$-$p\pi$ back bonding arises due to empty orbital of boron and filled orbitals of halogen.
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This $p\pi$-$p\pi$ back bonding has maximum effect in $BF_{3}$ as the size of B and F-atoms are comparative and this effect decreases as the size of halogen increases. Due to this effect, tendency of accepting lone pair of electron of boron decreases $i.e.$, Lewis acid character decreases.