Which space station utilized many leftover parts from the Apollo missions?

The Skylab space station, which was the United States' first space station, made extensive use of leftover components and materials from the Apollo program. This included hardware such as the Apollo's Saturn V rocket and various scientific instruments developed for Apollo missions. Skylab represented a transition in space exploration, building on the achievements of Apollo while also utilizing resources that were available after the conclusion of the Apollo program. This approach not only showcased innovation in repurposing existing technology but also allowed NASA to conduct scientific experiments and gather data in a microgravity environment at a reduced cost.

In contrast, the other options do not share this specific connection to the Apollo program. Mir was built by the Soviet Union and utilized entirely different technologies, while the International Space Station integrates elements from various countries’ space programs since its inception in the late 20th century. Tiangong is China's program and is distinct in its design and technology from earlier missions.