Question

how does the structure of this sonnet most clearly affect its meaning and style?
its rhyme scheme departs from a sonnets usual pattern in order to imitate the uniqueness of the birds song.
its regular pattern of rhythm, or meter, reflects the progression of one season to the next.
it has fourteen lines, which suggests the theme that the passage from youth to old age is brief.
it has a group of lines without stanza breaks, which matches the image of \solid tree trunks\ in the poem.

Explanation:

A sonnet typically has a regular meter (iambic pentameter). If the sonnet has a regular pattern of rhythm (meter), it can reflect a natural progression like seasons changing. A rhyme scheme departure (first option) is not a typical sonnet feature for imitating a bird's song. Fourteen lines (third option) don't directly suggest youth - old age brevity. A group of lines without stanza breaks (fourth option) is not a key sonnet - meaning connection compared to meter reflecting season progression.

Answer:

Its regular pattern of rhythm, or meter, reflects the progression of one season to the next.