A voice loud enough to be heard

The correct answer is A. Audible.

Audible means able to be heard. It is an adjective that describes something that can be perceived by the sense of hearing. For example, a loud noise is audible, while a whisper is not.

The other options are not correct because they do not mean able to be heard.

• Applaudable means worthy of praise.
• Laudable means deserving of
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  praise or commendation.
• Oral means spoken, not written.

I hope this helps!