The most appropriate one-word substitution for the given group of words "An act of committing a serious crime such as murder etc." is "Felony."

Explanation of the options:

☀ Hideous: This term describes something extremely ugly or unpleasant, but it does not specifically relate to the act of committing a serious crime.

☀ Criminal: This term refers to a person who has committed a crime or is involved in criminal activities, but it does not specifically capture the idea of a serious crime itself.

☀ Mafia: This term refers to a secretive and organized criminal group, typically involved in illegal activities such as extortion and drug trafficking, but it does not encompass the specific act of committing a serious crime.

☀ Felony: This term refers to a serious crime that is typically punishable by imprisonment for more than one year or by death. It accurately captures the meaning of the given group of words.