The suggested answer is C.
Requests for embedded content usually include the address of the site from which the request originated (this is known as the "referer" field in the request's HTTP header). This header information is used to determine categorization of the referred content.
Reference -
https://www.cisco.com/c/en/us/td/docs/security/wsa/wsa11-0/user_guide/b_WSA_UserGuide/b_WSA_UserGuide_chapter_01100.html