Final answer:
John Keats' poetry predominantly focuses on nature, aligning with the core themes of the Romantic Era, which highly valued natural beauty and the sublime.
Step-by-step explanation:
The poetry of John Keats is often associated with the Romantic Era, which was characterized by its glorification of nature, love, mythology, and the historical past. Keats' works conveyed a deep appreciation for the natural world, the complexities of love, and the allure of classical mythology. His poems, such as Ode on a Grecian Urn, are exemplary of the Romantic interest in beauty, emotion, and the sublime. While Keats' poetry touched on various themes, he is best known for his explorations of nature and beauty, making the correct answer to the student's question (a) Nature.