An acid is a substance which accepts:

Explanation

The correct answer is that an acid is a substance which accepts an electron pair, according to the Lewis concept. This concept broadens the traditional definition of acids beyond the Bronsted-Lowry theory, which focuses on proton donation. Thus, Option A is correct. Option B is incorrect as it describes the action of acids in the Bronsted-Lowry concept, which involves donating, not accepting, a proton. Option C is incorrect because it misinterprets the Lewis concept, emphasizing that acids accept electron pairs instead of single electrons. Option D is incorrect as no accepted theory or concept describes acids as accepting pairs of protons; the focus remains on single proton donation in the Bronsted-Lowry framework.