Gramps gifted a parcel of real estate to his grandson Chuck, with a provision written into the related deed that allows Gramps to cross the parcel over a specified path for as long as Gramps lived. The right retained by Gramps to so use the path is best legally characterized

a. as adverse possession of the path by Gramps

b. as an easement by grant from Gramps

c. as an easement by reservation by Gramps that exists as an easement in gross while Gramps is alive

d. as an easement by prescription created as a life estate