Select the option that can be used as a one-word substitute for the given group of words.
An unpleasant mixture of loud sounds.

The correct option is Cacophony.

Let's understand the meaning of the given words and see why Cacophony is the correct one-word substitute:

1. Cacophony: Refers to a harsh, discordant, or unpleasant mixture of loud sounds (e.g., traffic noise, sirens, and shouting all at once).

2. Harmony: Means a pleasing arrangement or combination of different sounds, notes, or voices, which is the exact opposite of an unpleasant mixture of sounds.

3. Hegemony: Refers to leadership or dominance, especially by one country or social group over others.

4. Irony: Refers to the expression of one's meaning by using language that normally signifies the opposite, typically for humorous or emphatic effect.

Therefore, the word that best substitutes "An unpleasant mixture of loud sounds" is Cacophony.