The most appropriate one-word substitution for the given group of words "An act of causing someone to become friendly again after an argument or disagreement" is "Reconciliation."

Explanation of the options:

☀ Sheath: A sheath is a protective covering or case for a blade or weapon and does not relate to the act of restoring friendship.

☀ Claque: A claque refers to a group of people hired to applaud or provide favorable reactions, typically in a theatrical or political setting, and does not pertain to reconciling after an argument or disagreement.

☀ Friendly: While "friendly" is a word related to being amicable or showing kindness, it does not capture the specific act of repairing a relationship after a disagreement.

☀ Reconciliation: This word precisely denotes the act of restoring friendship, harmony, or agreement between individuals who were previously in conflict or disagreement. It accurately reflects the meaning of the given group of words.