Why is the fact that light travels at 300,000 km/s is a consequence of energy conservation?

Answer is: true.

The law of conservation of energy states that the total energy of an isolated system remains constant; energy can neither be created nor destroyed; it can only be transformed or transferred from one form to another.

Albert Einstein (1879-1955) proposed that a beam of light is a collection of discrete wave packets (photons) with energy hν, where h is Planck constant and ν is frequency.

E = h·ν.

i think   the answer is c hope i'm right