Which statement best defines a Lewis acid?

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The essential idea is that a Lewis acid is defined by its ability to accept an electron pair. In Lewis theory, an acid is an electron-pair acceptor and a base is an electron-pair donor, so the correct statement directly captures that defining feature. A classic example is boron trifluoride, which has an empty orbital and accepts a lone pair from a base like ammonia to form a coordinate covalent bond. The other ideas describe different concepts: donating an electron pair identifies a Lewis base, and Bronsted-Lowry concepts describe proton transfer (a Bronsted-Lowry base accepts a proton, and a Bronsted-Lowry acid donates one). Some substances can be both Lewis and Bronsted acids or bases, but the Lewis definition centers on electron-pair acceptance.