The correct answer is (c), vacuum. Light travels at a maximum speed of 299,792,458 meters per second in a vacuum. This is the speed of light in a vacuum, and it is the fastest speed at which information can travel through the universe.
Light travels slower in air and water than it does in a vacuum. This is because light interacts with the atoms and molecules in air and water, which slows it down. The amount of slowing depends on the density of the medium. Air is less dense than water, so light travels slower in air than it does in water.
(a) Air is a gas, and light travels slower in gases than it does in solids or liquids. This is because the molecules in a gas are further apart than the molecules in a solid or liquid. The further apart the molecules are, the more time it takes for light to travel between them.
(b) Water is a liquid, and light travels slower in liquids than it does in solids. This is because the molecules in a liquid are closer together than the molecules in a solid. The closer together the molecules are, the more time it takes for light to travel between them.
(d) None of these is the correct answer. Light travels at a maximum speed in a vacuum.