D has the fastest reaction rate because dynamite explodes almost immediately.
A isn't correct because rusting nails take forever to completely rust.
B isn't correct because logs burning can take around 10 to 20 minutes.
C isn't correct because food digestions takes anywhere between 10 minutes to an hour.
Option D is indeed overall the best answer choice of the given options above.
The main reason given is that with an explosion, things happen quicker than the other options listed.