The correct answer is that a compound that donates protons.
On the basis of Bronsted-Lowry concept, a compound which accepts proton is considered as a base and the compound which donates protons is considered to be an acid. The strong acids and bases get ionized completely in aqueous solution, while the weak acids and weak bases get ionize partially.
The conjugate base is illustrated as the species that is produced after the loss of proton of acid, while the conjugate acid refers to the species that is produced after the gain of protons.
Thus, the Bronsted-Lowry definition of an acid is a compound that donates protons.