A Brønsted-Lowry acid is a substance that can donate a proton (H⁺ ion) to another substance. Among the options provided:
A. NH₄⁺ (ammonium ion) can donate a proton, so it acts as a Brønsted-Lowry acid.
B. NH₃ (ammonia) can accept a proton, so it acts as a Brønsted-Lowry base.
C. F (fluorine) can accept a proton, so it acts as a Brønsted-Lowry base.
D. BF₃ (boron trifluoride) cannot donate a proton, so it is not an example of a Brønsted-Lowry acid.
Therefore, the correct answer is A. NH₄⁺.
