When preparing a returned phone, what action ensures that previous user data is not accessible?

Erasing all content and settings is the most effective action to ensure that data from a previous user is not accessible on a returned phone. This process wipes the device clean, removing all personal information, apps, accounts, and custom settings. By completing this feature, you return the phone to the state it was in when it was first unboxed, which is crucial for protecting the privacy of the former user.

Changing the lock screen password does not delete any data; it simply modifies access security, leaving all user data intact. Reinstalling the operating system may not fully erase all user data, as remnants might remain even after a fresh install. Factory resetting the battery does not apply directly to data management and does not address user data stored on the phone itself. Therefore, the most thorough method for securing and preparing a returned phone is to erase all content and settings.