The correct answer is D. vacuum.
Light travels at the fastest speed in a vacuum, which is 299,792,458 meters per second. This is the speed of light in a vacuum, and it is the fastest speed at which any information can travel in the universe.
Light travels slower in other materials, such as glass, water, and hydrogen. This is because the light waves interact with the atoms and molecules in the material, which slows them down.
The speed of light in a material is given by the equation $v = \frac{c}{n}$, where $c$ is the speed of light in a vacuum and $n$ is the refractive index of the material. The refractive index is a measure of how much light slows down in the material.
The refractive index of glass is 1.52, the refractive index of water is 1.33, and the refractive index of hydrogen is 1.41. This means that light travels 1.52 times slower in glass than in a vacuum, 1.33 times slower in water than in a vacuum, and 1.41 times slower in hydrogen than in a vacuum.