:The correct answer is
The entire IP packet
Explanation:
In IPSec Tunnel Mode, the entire original IP packet (both the header and the payload) is encapsulated and protected by IPSec. A new IP header is added to the packet for routing purposes, while the original packet is encrypted and authenticated, providing confidentiality, integrity, and authenticity for the entire packet.
Payload only: This describes Transport Mode, not Tunnel Mode.
Header only: IPSec Tunnel Mode protects both the header and the payload, not just the header.
No answer is correct: This is incorrect because Tunnel Mode indeed protects the entire IP packet.