Correct Answer: Inaudible

Solution : The correct choice is the first option .

The word inaudible refers to a sound that is not able to be heard. In this context, it serves as a single word that succinctly captures the meaning of "a sound that cannot be heard".

The meanings of the other words are as follows:

• Audacious : This word describes someone bold, daring, or fearless.
• Autonomous : This term refers to something that operates independently or can function without external control.
• Delible : This word pertains to something that can be erased, blotted out, or easily removed.

Therefore, the most appropriate one-word substitute is inaudible.