Best way to ground an Intersystem Bonding Termination (IBT)?
D) None of the above

Most purpose-made Intersystem Bonding Termination devices are designed with a lay-in lug as part of them, designed to connect to a wire-type Grounding Electrode Conductor without disrupting the GEC, which is required by NEC 250.64(C) point 1 to be unspliced save for exothermic welds or irreversable compression splices.  As thus, I would do just that, mounting the device at a convenient point along the GEC and routing the GEC through the lay-in lug to make the connection.

If that is not an option, I would use a Burndy BDT mounted to the service equipment cabinet, with a minimum 6AWG bare copper bonding jumper inside the enclosure connecting the device to a ground bar.  This avoids having to reroute the GEC itself while still providing a convenient intersystem bonding termination point.