A treatise is a formal, usually lengthy, written work about a subject.

• A. a formal, usually lengthy, book or piece of writing about a particular subject
• B. a diary
• C. a short article
• D. a poem

Answer: (A)

A treatise signals a formal, careful, in-depth written work that tackles a subject in a systematic and substantial way. Because it aims to present arguments, evidence, and detailed analysis over a lengthy discussion, the description that best fits is a formal, usually lengthy book or piece of writing about a particular subject. A diary focuses on personal daily entries and experiences, not on formal analysis. A short article is concise and aimed at quick reading rather than thorough, in-depth examination. A poem uses verse and imagery rather than a formal prose exposition. So the description that matches a treatise is the formal, usually lengthy written work about a subject.