Which of the following is not true about fossil fuels

a. Some were created 300 mya
b. They were created from the full decomposition of prehistoric plants
c. They are non-renewable
d. They are composed of carbon

I think its B, because maybe not all of them were fully decomposed and thats how the fuel was created?

Its B.... prehistoric plants do not have to fully decompose... fossil fuels get their start as peat, which is like a crushed, partially decomposed plant... this peat will get buried, and through heat, pressure and time turn into fossil fuels such as coal

A is true because a good percentage of fossil fuels originated 300 mya in the Carboniferous period (literally "coal bearing"), a time when Earth was covered in warm, wet swampy forests
C is true because once you burn a fossil fuel you can't get it back
D is true also

Which of the following is not true about fossil fuels?
a. Some were created 300 mya
==> b. They were created from the full decomposition of prehistoric plants
c. They are non-renewable
d. They are composed of carbon
--------------------------------------…
You are correct in your thinking. If the prehistoric plants were fully decomposed, there would be nothing left of them from which to form the fossil fuels.
============
I hope this is helpful.