Answer
Methyl red is the most suitable, because it changes its color in acidic solutions, and the equivalence point of the titration between $NH_3$ and $HCl$ is in acidic solution.
Work Step by Step
$NH_3$ is a weak base and $HCl$ is strong acid; therefore, the pH at the equivalence point of their titration will be acid $(pH < 7)$.
Analyzing the 3 given indicators on page 667, the one where the color changes in acidic solution is the most appropriate, so Methyl red is the correct answer.
