Explanation and answer:
A solution containing monosodium phosphate and disodium phosphate is a buffer solution.
A buffer is made by mixing a large volume of a weak acid or weak base together with its conjugate base or acid.
How would this pair neutralize a base?
H₂PO₄- + H2O <---->
NaH₂PO₄ + NaOH <----> Na₂HPO₄ + H₂O
