nothing can __________ such appalling behavior.

remonstrate, extenuate, or supplication

remonstrate

Nothing can remonstrate such appalling behavior.

To determine the correct word that can fill in the blank and appropriately complete the sentence, let's examine the given options and their meanings:

1. Remonstrate: This verb means to forcefully protest or argue against something, usually in an effort to persuade or criticize. It does not fit in the sentence because it suggests a reaction against the behavior, rather than implying an equivalence to the behavior itself.

2. Extenuate: This verb means to make a wrongdoing appear less serious or to provide a justification that lessens the blame. It is also unsuitable for the sentence because it implies an attempt to diminish the severity of the behavior, which contradicts the idea of "appalling" behavior.

3. Supplication: This noun means a humble plea or request for something, often in a submissive or prayerful manner. While the word conveys a sense of begging or pleading, it does not align with the sentence, as it does not capture the concept of nothing being able to match the appalling behavior.

Therefore, the correct word that can fill in the blank and make the sentence logical is "extenuate." The sentence would read as follows: "Nothing can extenuate such appalling behavior."