Final answer:
Gravitational waves travel at the speed of light, which is a constant that does not depend on their frequency or wavelength. The speed of sound is determined by the properties of the medium, not the wave's frequency or wavelength. The speed of light in a vacuum is a fixed constant, unaffected by factors such as reflection or hypothetical media like the ether.
Step-by-step explanation:
Gravitational waves propagate at the speed of light, which is a constant speed in a vacuum (option a). This speed does not depend on the frequency or wavelength of the gravitational waves. Therefore, the correct answer is: a) At the speed of light.
The speed of sound, however, does depend on the properties of the medium through which it is traveling, not on the frequency or wavelength (option d). Unlike gravitational waves, the properties of the medium, such as its temperature, density, and state (solid, liquid, or gas), significantly affect the speed of sound.
For electromagnetic waves, including light, the propagation speed in a vacuum is a constant, known as c, and is not dependent on the motion relative to any medium like the hypothesized ether (option b for electromagnetic waves). The speed of light also does not change upon reflection (option c for electromagnetic waves), and in a vacuum, it is exactly 3.00×108 m/s (option d for electromagnetic waves).