Which of the following refers to the unique way an author uses language?

The term that refers to the unique way an author uses language is "voice." An author's voice encompasses their individual style, which includes choices in diction, tone, and syntax, as well as the rhythm and flow of their writing. Voice is what makes an author's work distinctive and recognizable, reflecting their personality and perspective.

Diction refers specifically to the choice of words and their connotations, while syntax pertains to how those words are arranged in sentences. Tone relates to the author's attitude toward the subject matter or the audience. While all these elements contribute to the overall voice, it is the composite of these aspects that truly defines an author's unique linguistic signature.