Which of the following is required when participating in a contest on HF frequencies?
Why Contest operating is just normal amateur operating with a score attached. FCC rules still apply in full, including the 10-minute identification requirement of 97.119, so you must send your call sign at least every 10 minutes during a contact and at its end. Contest sponsors may ask for logs or encourage QSLing, but those are the sponsor's rules or courtesies, not FCC requirements.
Watch out Submitting a log is a condition for having your score published by the sponsor, and QSLing is entirely voluntary; neither is anything the FCC requires.
Contests change your operating style, not the rules: ID every 10 minutes, same as always.