The speed of light in a vacuum is exactly 299,792,458 metres per second.
Why it is true
Since 1983, the speed of light in a vacuum has been a constant defined by convention. The exact value is 299,792,458 metres per second. From it, the metre came to be defined as the distance light travels in 1/299,792,458 of a second.
Source: BIPM
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.