A linking verb, as the name itself says, is used to link, or to connect parts of a sentence. In this sentence, there are two verbs - went and is. However, went is not a linking verb but rather a main verb. This means that the linking verb is IS.
<span>the power of nature
In the passage the speaker talks about how the ground swells due to the water in the soil freezing. It is so strong that it lifts the boulders and breaks apart the wall. The speaker's use of the word "something" and the description of how the ground, or nature, destroys the wall each year emphasizes the power of nature.
The other choices can be eliminated because the stones don't actually break - they're just relocated. Also, there is no mention of a person in this section of the poem at all.
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