Answer:
A) An insect nearly microscopic in size is responsible for felling a forest of conifers.
Step-by-step explanation:
Remember that irony is a figure of speech in which words that are used intend to have a different meaning other than the literal one. A difference between appereance and reality is made. One situation is shown to have contradictory meanings.
In this case the forest is huge and one could think that to destroy it, it is necessary a huge object or animal. Instead a tiny, almost microscopic insect is the one responsible for it's felling. The author even emphasizes it's size by writing, "smaller than the period at the end of this sentence."