Final answer:
The bond formed between nitrogen (N) and boron (B) when NH3 reacts with BF3 is a coordinate covalent bond, where the nitrogen atom donates its lone pair of electrons to the boron atom.
Step-by-step explanation:
The bond formed between nitrogen (N) and boron (B) when a molecule of NH3 reacts with a molecule of BF3 is a coordinate covalent bond. In this type of bond, both bonding electrons are donated by one of the two bonded atoms. In this case, the lone pair of electrons on the nitrogen atom is donated to the boron atom, forming a coordinate covalent bond.