Final answer:
The poem 'I heard a fly buzz when I died' mostly adheres to a regular pattern of meter with an irregular rhyme scheme, making option C the correct description of its structure.
Step-by-step explanation:
The sentence that best describes the poetic structure of the poem 'I heard a fly buzz when I died' is option C: The poem mostly follows a regular pattern of meter, and it has an irregular rhyme scheme. This poem is not written in free verse as it does not entirely abandon metrical patterns or rhyme. Instead, Emily Dickinson's poem employs a loose iambic meter with variations, and the rhyme scheme is not strict but features occasional slant or near rhymes. While it is not in free verse, it also does not follow a completely strict metrical pattern or rhyme scheme; thus, it maintains some rhythm and structure, but with notable irregularities that prevent it from being categorized as strict or regular.