Answer
Bromthymol blue is the most suitable, because it changes its color in neutral solutions, and the equivalence point of the titration between $HNO_3$ and $KOH$ is in neutral solution.
Work Step by Step
$HNO_3$ is a strong acid and $KOH$ is a strong base; therefore, the pH at the equivalence point of their titration will be neutral $(pH = 7)$.
Analyzing the 3 given indicators on page 667, the one where the color changes in neutral solution is the most appropriate, so Bromthymol blue is the correct answer.
