Which of the following designs is the most satisfactory method for attaching turbine blades to the rotor disk?

Correct Answer: Option C — Fir-tree design.

The fir-tree design is the industry standard for attaching turbine blades to rotor disks. Its geometry provides a large surface area for load distribution, effectively managing the immense centrifugal forces while allowing for thermal expansion and vibration damping, which is essential for the longevity of high-speed turbine components.