Final answer:
BF₃ is a Lewis acid but not a Brønsted-Lowry acid. HCl, H₂O, and NH₃ can act as both Lewis acids and Brønsted-Lowry acids.
Step-by-step explanation:
The Lewis definition of an acid is a substance that can accept an electron pair, while the Brønsted-Lowry definition of an acid is a substance that can donate a proton. Based on these definitions, a Lewis acid that is not a Brønsted-Lowry acid is BF₃. This is because BF₃ can accept an electron pair from a base, but it cannot donate a proton. The other options (HCl, H₂O, and NH₃) can all act as both Lewis acids and Brønsted-Lowry acids.