What are the two common forms of blade roots?

Correct Answer: Option D — Fir tree and bulb.

Blade roots secure turbine and compressor blades to the rotor disc. The fir tree design provides high strength and stress distribution, while the bulb root offers a simpler, robust attachment method. Both are industry-standard configurations designed to withstand the extreme centrifugal forces and thermal expansion encountered during high-speed turbine operation.