The correct answer is B.
The study guide explicitly states: "IKE version 2 does not interoperate with IKE version 1, but they share enough of the header format that both versions can unambiguously operate over the same UDP port." That directly proves B.
Why the other options are wrong:
A is wrong because the study guide shows authentication methods as Asymmetric for IKEv2 and Symmetric for IKEv1 C is wrong because the study guide does not say they use the same TCP port; instead, it specifically says they can operate over the same UDP port D is wrong because the study guide states: "IKEv2 does not use the concept of phase 1 or phase 2", even though FortiOS CLI/GUI still uses those terms for configuration purposes