Just in case our answer choices are different for some reason, the correct answer is: "It is used as evidence against the contention that the Forestry Department is not doing its job properly if it does not extinguish forest fires as quickly as possible."
This tells us that letting the small fires burn is in the best interest of the Forestry Department, because it prevents large, devastating fires. Therefore, the department is doing its job properly, assuming that the priority is to defend the forest from devastation. (We can safely assume that.)
I suppose the consequences from C refer to devastation of the forest that would occur from a large fire. However, the problem with this answer choice is "most people." The passage only states that "many people" think that fires must be extinguished as soon as possible. We do not know the ideas of most people, therefore the purpose of the given statement is not to demonstrate the consequences of those ideas.