The correct answer, "a chapter of his novel Rasselas," is linked to Samuel Johnson’s famous work titled The History of Rasselas, Prince of Abissinia (1759). In this philosophical tale, Johnson discusses various aspects of human life, including poetry. The "Dissertation upon Poetry" is not a formal standalone work of Johnson's but refers to a section of Rasselas that is framed as a reflective and philosophical discussion about the purpose and value of poetry.
In Rasselas, there is a chapter where Johnson explores poetry's role, its capacity to offer wisdom, and its limitations when confronting real human suffering and complexities. This dissertation-like treatment of poetry within the novel resembles the tone and structure of Johnson's essays and critical writings, but it is embedded in the larger philosophical narrative of Rasselas.
Hence, the correct answer is option 2 because the "Dissertation upon Poetry" forms part of the broader discourse in Rasselas.