Hey there!
Let's remember something. All over us, there is pressure, everywhere we go. So, this means that, if you were to hold something, (especially) high off the ground and there is higher pressure in the sky then on level ground, this would mean that the molecules would be more complex, and they would break down.
So, from your options listed above, I believe that the best answer to would go with this question would be Option B.) They break down complex molecules.
I hope this helps you!