What refers to various sentence structures, styles, and lengths that enhance the rhythm or emphasize a piece of text?

The correct choice is based on the concept of enhancing a text's effectiveness through the use of different sentence structures, styles, and lengths. This variety keeps the writing engaging and dynamic, allowing for a rhythm that can evoke different emotions and emphasize key points within the text.

When writers implement sentence variety, they deliberately mix short, punchy sentences with longer, more complex ones. This not only aids in maintaining the reader's interest but also helps to underscore particular elements of the narrative or argument being presented. For instance, a sudden short sentence in a primarily longer paragraph can create a dramatic impact, drawing the reader's attention to an important idea or moment.

Other options do not capture this specific idea of mixing different styles and lengths to create a rhythmic flow in writing. Lexical variation refers to the use of diverse vocabulary rather than sentence structure. Textual rhythm could imply an understanding of how text flows but lacks the focus on the diversity of sentence forms specifically. Prose styles relate more to the overall tone and formality of writing rather than the construction of individual sentences that contribute to rhythm and emphasis. Therefore, the emphasis on varying sentence structure and style makes "sentence variety" the correct answer.