One word substitution: choose the word that can be substituted for the given description. Someone who is incapable of being quietened or pacified.

Introduction / Context:
This question tests your knowledge of one word substitutions, a common area in competitive English. You have to match a description with the precise term that expresses the same idea in a single word.

Given Data / Assumptions:

• Description: Someone who is incapable of being quietened or pacified.
• Options: Implacable, Unflappable, Insatiable, Inexplicable.
• The required word must describe a person who cannot be calmed down or satisfied.

Concept / Approach:
Implacable refers to a person or force that cannot be appeased, pacified, or changed. It is often used for implacable anger, implacable hostility, or an implacable enemy. The key idea is that no amount of pleading or negotiation will calm this person. This directly fits the description given in the question.

Step-by-Step Solution:
Step 1: Focus on the central idea in the description: cannot be quietened or pacified. Step 2: Recall that Implacable is used when feelings like anger or hatred cannot be reduced. Step 3: Compare the other options and see that they describe different qualities. Step 4: Choose Implacable as it best matches the sense of being impossible to calm.

Verification / Alternative check:
Dictionaries define Implacable as unable to be placated, where placate means to make someone less angry or hostile. This perfectly matches being incapable of being quietened or pacified. Therefore Implacable is the only option that fully covers the description.

Why Other Options Are Wrong:

• Unflappable: Describes someone who stays calm and does not get upset easily, the opposite situation from a person who cannot be pacified.
• Insatiable: Refers to someone whose desires or appetites cannot be satisfied, often used with hunger or curiosity, not specifically about pacifying emotions.
• Inexplicable: Means something that cannot be explained, unrelated to calming someone.

Common Pitfalls:
A common error is to choose Insatiable because it also uses the idea of cannot be satisfied. However the question is about calming or pacifying a person's emotional state, not about satisfying hunger or desire. Understanding these fine shades of meaning is essential for scoring well in one word substitution questions.

Final Answer:
Correct option: Implacable.