What tool is used to hold the head of the ground stud from turning while the nutrunner tightens the locking nut?

The correct choice is the dog bone wrench, which is specifically designed for applications where the head of a ground stud needs to be secured in place while a nutrunner tightens the locking nut. This tool typically has two openings of different sizes to fit various fasteners securely and provides a stable grip on the ground stud without risking damage.

Using a dog bone wrench allows the mechanic to effectively prevent the ground stud from rotating during the tightening process, ensuring that the nut is properly secured and that the fastener's integrity is maintained. This feature is particularly important in aerospace applications, as the safety and reliability of assemblies depend on tight and secure fastenings.

The other tools listed, such as flat wrenches, adjustable wrenches, or torque wrenches, do not have the same design specificity or functionality as the dog bone wrench for this particular scenario. Flat wrenches and adjustable wrenches may lack the secure grip needed to stabilize the stud, while torque wrenches are primarily used for measuring the amount of torque applied to a fastener rather than holding it stationary.