Which of the following is required when participating in a contest on HF frequencies?

Participating in an amateur radio contest does not exempt an operator from the standard FCC rules. You must still properly identify your station at least every 10 minutes and at the end of a communication, just as you would during any normal QSO. While contest sponsors have their own rules regarding log submissions, and many operators choose to send QSL cards or use Logbook of The World (LoTW), these are voluntary activities, not legal mandates. The FCC does not care if you submit a contest log or send QSL cards. Therefore, the only true legal requirement among the choices provided is to abide by standard station identification regulations.