So the idea for question 15 is to identify that the reactants shouldn't be completely reacted off to create an equilibrium (as only water will be left, which is not a buffer). With this, consider the eqn:
CH3COOH(aq) + OH−(aq) → CH3COO−(aq) + H2O(l)
Mole ratio: 1:1 to completely react each...