What must be done to hearing aids before they are tested?

Before conducting tests on hearing aids, resetting them to the manufacturer's settings is necessary to ensure accurate and reliable results. This process restores the device's original configuration and eliminates any user adjustments or personalized settings that may affect the test outcomes. By returning the hearing aid to its baseline state, clinicians can evaluate its performance as designed and intended by the manufacturer.

The other choices highlight important considerations in the maintenance and preparation of hearing aids, but they do not specifically address the necessity for ensuring that the device operates under standardized conditions during testing. While cleaning, replacing batteries, and charging the devices are essential for proper functionality and performance during regular use, they do not provide the same foundational assurance of uniformity and objectivity in test environments as resetting to manufacturer settings does.