Can diamond become graphite spontaneously

Yes

Let's start by clarifying that both diamond and graphite are formed by the same carbon atoms, but packaged differently.

Therefore the graphite is used to create artificial diamonds, by putting it in conditions of high temperature and pressure (a pressure equivalent to thousands of times the Earth's atmosphere) can cause atoms to change their packaging and become a diamond.

The reverse process is also possible, also under the influence of high temperatures (thermal energy) a diamond can be converted back into graphite.

t = 1/400 h   ⇔ 0.0025 h ⇔ 9s ⇔ 0.15 m

any of the answers are correct.