The question says, "It can be inferred that a forest burned as described in the passage would have been LEAST likely to display..."
We're looking to pick the answer that is the least likely result of controlled burning. In looking at the third paragraph, we know that controlled burning converts "mixed stands of trees to homogenous forests" (lines 37-38). Thus, we would have the expectation that controlled burning would produce forests with only a single type of tree, not numerous types of hardwood trees. Since we're looking for the least likely answer, (A) is the opposite of what we would expect, so it's the least likely and is the correct answer choice.
Does this make sense? Let us know if you have any other questions!