Which of the following indicates that a hearing aid is working properly?

A whistling sound indicates that a hearing aid is working properly because it often signifies that the device is amplifying sound effectively. This feedback is usually produced when sound is escaping the hearing aid and reverberating, which can happen due to the fit of the hearing aid or its placement in relation to the ear canal. While it might be uncomfortable for the user, this feedback demonstrates that the hearing aid is, at least in part, functioning as intended by amplifying sound.

In contrast, a constant hum or a skipping sound suggests issues with the device. A constant hum could indicate a problem with the hearing aid's electronics, while a skipping sound points to irregular sound processing. Additionally, no sound at all signifies that the hearing aid may not be operational, indicating a need for troubleshooting or adjustments, such as checking the batteries or ensuring the device is turned on. Thus, the whistling sound is a key indicator of functionality in a hearing aid, while the other options reflect potential faults or malfunction.