Which of the following shops is primarily involved in structural repairs?

The Structure Shop is primarily involved in structural repairs because it focuses on the maintenance, modification, and repair of the aircraft’s airframe and other structural components. Technicians in this shop are trained to handle a variety of tasks that ensure the aircraft's structural integrity, which may include replacing damaged sections, reinforcing weak areas, and conducting inspections to meet safety standards.

The work carried out in the Structure Shop is critical to maintaining the overall safety and operational capability of the aircraft. It involves various materials and techniques necessary for ensuring the structural soundness of airframes, which is essential for the aircraft's performance and longevity.

While the other shops mentioned have their specific functions—such as the Machine Shop focusing on manufacturing and repairing parts, the Paint Shop specializing in surface finishing and aesthetics, and the Welding Shop dealing with metal joining and repair—the primary responsibility for structural repairs resides in the Structure Shop. This distinct focus makes it the correct choice for this question.