An acid can be defined as
a. a proton acceptor.
b. a proton donor.
c. neither a proton donor nor a proton acceptor.
d. both a proton donor and a proton acceptor.

b. a proton donor.

Explanation:

An acid can be defined as a substance that produces hydrogen ions or H+ protons when dissolved in water, according to Arrhenius

It, also, can be defined as a molecule or ion which gives or donates a H+ protons when dissolved in water, according to Bronsted y Lowry

And, finally, according to Lewis, an acid can be defined as any compound that can accept a pair of electrons.