I lean toward (b). If the beneficiary put forth a good argument in favor of ©, I might be sympathetic. Of course, if the PA has a precedent or administrative practice, that should be included.
While Effen's suggestion of an escrow account might be nice, the PA still has to account for the possibility that the designated beneficiary will never show up, or is currently deceased, in which case the PA (and/or the plan) may have procedures for designating a contingent beneficiary.