Under which circumstance is the IPsec ESP traffic encapsulated over UDP?

Under which circumstance is the IPsec ESP traffic encapsulated over UDP?

Response:
A . When using IKE version 2 (IKEv2)
B . When the phase 1 is configured to use aggressive mode
C . When the IPsec VPN is configured as dial-up
D . When NAT-T detects there is a device between both IPsec peers doing NAT over the IPsec traffic

Answer: D