# $NH_3$ and $BF_3$ form an adduct readily because they form

$\begin{array}{1 1}(a)\;\text{an ionic bond}&(b)\;\text{a hydrogen bond}\\(c)\;\text{a covalent bond}&(d)\;\text{a coordinate bond}\end{array}$

This is co-ordinate bond between N and B.
Hence (d) is the correct answer.