Answer: His melancholy tone highlights the despair of the excerpt.
Explanation: In the given excerpt from Irving's "The Mysterious Stranger" we can see the description of a man struggling with strong emotions, his gestures (like clasping his hands together until his fingers were imprinted in his flesh, or sinking his face upon his knees) are signs of despair and suffering. So the statement that best describes the tone in the excerpt is: His melancholy tone highlights the despair of the excerpt.