Which of the following actions can be categorized as altering theft prevention devices?

Altering theft prevention devices refers to actions that interfere with the functionality or intended use of security systems designed to prevent shoplifting or theft. Removing a price tag can be seen as altering a theft prevention device because price tags often include security mechanisms, such as ink tags or barcodes, that trigger alarms when items are taken from the store without being properly checked out. By removing a price tag, a person could potentially defeat these security measures, making it easier to steal the item.

The other choices do not fall under the category of altering theft prevention devices. Returning stolen items does not alter the device itself; it simply involves giving back the item. Purchasing security tags is a preventative measure that does not physically alter existing devices. Thus, the action of removing a price tag is the one that clearly qualifies as altering a theft prevention device.