Answer: d. the drill rig maintenance firm.
Step-by-step explanation:
The drill rig maintained firm are not responsible for any defects in the drill because they did not manufacture it so they cannot be sued by Elmer for any injury.
The drill rig lessor however, can be sued for potentially leasing a defective rig to DDC without checking it and DDC could be liable as well for letting their employees work with such a rig when it wasn't adequately checked yet. The manufacturer of the rig can be sued for the reason of potentially making a defective rig.