Answer:
D. when the number of moles of acid is exactly equal to the number of moles of base.
Step-by-step explanation:
Regarding options A. and E., pKa and pKb would only be taken into consideration if the titrations were of weak acids and bases. However it is possible to have a titration of monoprotic acids and bases with strong acids and bases.
Another way of looking at the answer is identifying which one best describes the equivalence point.