Which characteristic of birch trees does robert frost explore in his poem "birches"?

If its multiple choice, the answer is Resilience.

Robert Frost explored the flexibility of the birch in his poem. The action of swinging is shown as an escape from the reality and hardship of life. For a boy swinging from the birch, it is an act of joy and freedom. For an adult, it is not possible to leave behind one's responsibilities.