Accumulation of which one of the following acids results in the...

According to $K^{+} - H^{+}$ pump theory during the closure of stomata, ABA stops the energy dependant proton pump in guard cell, thus $K^{+}$ diffuses out of the cell and $H^{+}$ enters inside, which combines with malate ion to form malic acid. OP of guard cells decreases thus stomata close.