Select the option that can be used as a one-word substitute for the given group of words.

Loud enough to be heard.

To find the one-word substitute for the phrase "Loud enough to be heard", let us examine the meanings of the given options:

1. Audit: An official inspection of an organization's accounts, typically by an independent body.
2. Audacity: A willingness to take bold risks, or rude/disrespectful behavior.
3. Audacious: Showing a willingness to take surprisingly bold risks, or showing an impudent lack of respect.
4. Audible: Able to be heard; loud enough to be heard. The word comes from the Latin root audire, which means "to hear".

Therefore, the word that perfectly substitutes the given group of words is Audible.

The correct option is Audible.