During the titration of a 0.100 M monoprotic weak acid with 0.100 M NaOH, a student adds base past the equivalence point so that the base is in excess. Which species present in the final solution primarily determines the pH?
a) The conjugate base A− produced during the neutralisation
b) The remaining weak acid HA in equilibrium with its ions
c) The excess OH− ions introduced by the strong base
d) Hydronium ions formed by the auto-ionisation of water