Which device amplifies sound to levels that are clearly heard by everyone?

The correct choice for amplifying sound to levels that are clearly heard by everyone is the amplifier. An amplifier is specifically designed to take low-level audio signals and increase their power so that they can drive speakers effectively, producing sound at levels audible across larger spaces or to larger audiences.

Microphones are used to capture sound, converting acoustic energy into electrical signals, but they do not amplify sound themselves. Speakers reproduce sound from the audio signals they receive, which need to be amplified first to be effective, and mixers combine multiple audio inputs but do not amplify a single signal on their own. Therefore, the amplifier plays a crucial role in ensuring that sound can be heard clearly by all, making it the essential device for achieving that goal.