The correct answer is D) Fir tree and bulb. The two common forms of axial compressor rotor blade roots are the fir tree and bulb designs. The fir tree root provides a secure attachment of the blade to the rotor disk, while the bulb root allows for easy installation and removal of the blade. This combination of the fir tree and bulb designs is commonly used in axial compressor applications. The other answer choices are incorrect as they do not accurately describe the typical axial compressor rotor blade root designs. Grub and bulb, sawtooth and knob, and fir tree and key are not the standard forms used in axial compressors.