The technique you’re referring to is known as C. Entity tagging.
Entity tagging, also known as ETag tracking, is a method where web publishers utilize data cached by a browser and returned with a subsequent request from the same resource to track unique users. This can serve as an alternative to third-party cookies, especially as their use is being reduced due to privacy concerns and regulations.
Please note that while this method can help with user tracking for analytics or personalization purposes, it also raises privacy concerns and should be used responsibly, in compliance with relevant laws and regulations.