What does 'inaudible' mean?

The term 'inaudible' refers specifically to a sound that is so quiet that it cannot be heard by the human ear. This definition aligns perfectly with the choice that indicates it is "too quiet to be heard." In the context of auditory perception, something described as inaudible implies that it lacks sufficient volume or intensity to be detected, making it impossible for listeners to perceive it clearly.

The other choices describe different qualities of sound—too loud to be ignored suggests a sound that is very noticeable, while clear and understandable refers to the clarity of speech or sound, and weak and unnoticeable conveys a sense of being faint but not necessarily silent. However, 'inaudible' specifically emphasizes the inability to hear due to lack of volume, which is captured by the idea of being "too quiet to be heard."