Lewis acid-base theory defines a Lewis acid as an electron-pair acceptor and a Lewis base as an electron-pair donor. In the reaction BF_3 + :NH_3 \rightarrow F_3B-NH_3, BF_3 has an incomplete octet on boron (only 6 electrons), making it an electron-pair acceptor — a Lewis acid. NH_3 has a lone pair on nitrogen that it donates to the empty orbital of boron, making it a Lewis base. Option A incorrectly assigns the acid role to NH_3; an electron-pair donor is a Lewis base, not a Lewis acid. This is a common student confusion. Option C is wrong because F_3B-NH_3 is the Lewis adduct (product), and the acid-base classification applies to the reactants before bond formation. Option D is wrong because in this reaction NH_3 donates electrons and BF_3 accepts; both cannot simultaneously be Lewis acids. The Lewis theory, broader than Brønsted-Lowry theory, is explicitly covered in NCERT Equilibrium chapter and is a favourite JEE conceptual question. Plausibility check: BF_3 is a well-known Lewis acid in organic chemistry (used as a catalyst) precisely because of its electron deficiency, confirming our classification.