The answer is: D.when preparing to measure pH change.
Near equivalence point indicator should change color, so we must pick indicator who change color near pH of equivalence point.
Equivalence point is the point which there is stoichiometrically equivalent amounts of acid and base.
For example, phenolphthalein is colorless in acidic solutions and pink in basic solutions. If this indicator change color to dark red, more base is added and endpoint is not accurate.
Chemist can draw pH curve (graph showing the change in pH of a solution, which is being titrated) for titration and determine equivalence point.