Final answer:
The Republic of Letters was a community of Enlightenment era writers and intellectuals who exchanged ideas across Europe and the Atlantic, representing an important historical sphere of information exchange.
Step-by-step explanation:
The student's question, 'The Republic of Letters refers to which sphere of information exchange,' relates to history, specifically within the context of intellectual and cultural exchanges during the Enlightenment. The correct answer to the question is option 'a': the Republic of Letters was a long-distance community of writers who corresponded with each other across Europe and the Atlantic. These writers and intellectuals exchanged ideas, critiqued each other's works, and disseminated knowledge during the eighteenth century, contributing to the spread of Enlightenment ideals