Boric acid is an acid because its molecule
(i) contains replaceable ion
(ii) gives up a proton
(iii) accepts from water releasing proton
(iv) combines with proton from water molecule
The answer is the option (iii) accepts from water releasing proton.
Boron has a small atomic size and only 6 electrons in the valence shell, releases a proton, when it accepts electrons from ion of.