Answer
A. An acid accepts an electron pair from a base.
Explanation
In Lewis's acid-base reaction theory, bases donate pairs of electrons and acids accept pairs of electrons. A Lewis acid is therefore any substance, such as the H⁺ ion, that can accept a pair of nonbonding electrons. -This means, a Lewis acid is an electron-pair acceptor.
Therefore, the correct answer is:
A. An acid accepts an electron pair from a base.