Which suspension component can only be used on unibody vehicles and is a combination spring/damper?

The Macpherson strut is the correct answer because it is specifically designed for use in unibody vehicles and serves as a combination of a spring and a damper. In a Macpherson strut setup, the strut assembly integrates both the coil spring and shock absorber into a single unit that helps support the weight of the vehicle while managing its suspension dynamics. This design is particularly effective in unibody construction as it provides a compact and efficient means of connecting the wheel assembly to the chassis.

The integration of the spring and damper within the strut simplifies manufacturing and installation, making it a popular choice for modern vehicles that require a lightweight and space-saving suspension system. The unibody design, common in many contemporary vehicles, benefits from this approach because it allows for better handling and performance characteristics without adding unnecessary weight.

The other options do not meet these criteria. Strut springs are simply a part of the strut assembly, while coil springs and leaf springs are separate components used in various suspension types but are not specifically designed as a combination unit for unibody vehicles. Leaf springs, in particular, are more suited to traditional body-on-frame vehicles and do not typically integrate with the damping function as seen in Macpherson struts.