Final answer:
The figure of speech in the given sentence is a Metaphor, which directly compares October's poplar trees to flaming torches without using 'like' or 'as'.
Step-by-step explanation:
The figure of speech used in the sentence "October's poplars are flaming torches lighting the way to winter" is a Metaphor. This is because the sentence directly compares poplar trees to flaming torches without using the words "like" or "as", which would indicate a simile. Metaphors are a type of figurative language that create vivid imagery by asserting that one thing is another, effectively highlighting the similarities between two different things and enhancing the descriptive quality of the text.