Alternative D: its Lower Reaches
First, focus on that portion of "D" running just north from the I-10 at Benson. The mountains located immediately west of the D label on this map are the Little Rincons, with the Rincons lying further west of them their eastern edge slashed by a sharp north-south fault (the Martinez ranch Fault), and the Happy Valley basin headwaters of the Paige Wash standing out in between.
Second, where the route leaves the I-10, the location appears to take off at the Skyline Interchange (exit 299) on the high terrace west of Benson, then runs north, then northeasward to a connection with the now-existing Ocotillo Road, which then runs north up the west side of the San Pedro, pasing by the Benson Narrows to its east. For details on the prospects of this route, see these side-pages: