Final answer:
Option d. 4-6 would be the most useful pH indicator range for the titration of a weak base with a strong acid.
Step-by-step explanation:
In general, for the titration of a weak base with a strong acid, an indicator with a pKin < 7.0 should be used. This is because the pH at the equivalence point for a weak base is less than 7.0. Among the given options, the indicator range that falls within this criteria is d. 4-6. Therefore, option d. 4-6 would be the most useful pH indicator range for the titration of a weak base with a strong acid.