Charlie aims to utilize IPSec and wants the ability to specify the IPSec policy for traffic based on the port it is being sent to on the remote system. What mode of IPSec should he choose?

In contrast to IPSec's tunnel mode, IPSec transport mode enables the use of varying policies per port. The policy applied to the packet is determined by the IP addresses in the outer header for transport mode packets. While IPSec does not offer a PSK mode, WPA-2 does. IKE is used to establish security associations in IPSec but does not allow this type of mode configuration.