The writer compares "leaves in wintry weather" to b.) memories of the past.
Through reflection and by bringing pleasant memories of friendship to the fore, the author has captured the idea of friendship in the poem. He reflects on his boyhood and the good and bad experiences that occurred during that period.
The central idea of a poem is its topic, or, if you prefer, "what it's about." Many people are uncomfortable with poetry being "about" something, but at the end of the day, the poet had something in mind when they wrote it, and that something—whatever it may or may not have been—is the main idea.
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