Which of the following words suggests a tone that is bitterly scathing?

The word "vitriolic" is the most fitting choice for suggesting a tone that is bitterly scathing. This term originates from the Latin word "vitriol," which refers to sulfuric acid, and is often used to describe comments or language that are harshly critical and filled with bitterness. It implies a level of venomous sharpness and intensity that communicates deep animosity or disdain.

In contrast, the other words do not convey this bitter and scathing tone. "Harsh" relates to something that is unpleasantly rough or severe, but it does not necessarily imply a scathing quality. "Selfless" denotes a lack of selfishness or concern for oneself, which is completely opposite to a bitter tone. "Conciliatory" suggests a tone that aims to placate or reconcile, indicating a desire to make peace rather than to criticize or attack. Thus, the choice of "vitriolic" distinctly captures the essence of bitterness and scathing criticism, making it the correct answer.