Conditionals (also called conditional or if clauses) are used to describe the result of something that might happen in the present or future or could have happened in the past but didn't. There are four main kinds of conditionals: zero, first, second, and third conditional.
The zero conditional is used when want to talk about facts or things that are generally true.
The first and second conditionals talk about the future. However, the first conditional is used for real possibilities, while the second is used for unreal possibilities.
The third conditional is used when we talk about the past, a condition from the past that did not happen. Thus, we can say that it is used when there are no possibilities of something happening.
You can see how different conditionals are built and easily recognized in the image below:
I agree with the person above - the correct answer should be A. resilience. The poem tells us a story of how birches are strong trees that can withstand anything. They will slump down, however, they will not break because they are resilient and can endure many things that happen to them, like children playing on them, or winds trying to break them.