A Solution Is Prepared by Dissolving 0 $K_{a}$ Of Formic Acid Is 1

A solution is prepared by dissolving 0.23 mol of formic acid and 0.27 mol of sodium formate in water sufficient to yield 1.00 L of solution.The addition of 0.05 mol of NaOH to this buffer solution causes the pH to increase slightly.The pH does not increase drastically because the NaOH reacts with the ________ present in the buffer solution.The $K_{a}$ of formic acid is 1.8 × 10^-4.

A) $\mathrm{H}_{2}O$
B) $\mathrm{H}_{3} \mathrm{O}^{+}$
C) formate
D) formic acid
E) This is a buffer solution: the pH does not change upon addition of acid or base.