Final answer:
d) Personal letters among intellectuals.
The Republic of Letters was a long-distance community of intellectual exchange among writers across Europe and the Atlantic, crucial for the dissemination of Enlightenment ideas.
Step-by-step explanation:
The Republic of Letters refers to a sphere of information exchange that functioned as a long-distance community of writers who corresponded with each other across Europe and the Atlantic in the seventeenth and eighteenth centuries. Participants of the Republic of Letters were united not by geographic location but by ideas spread through print. They were prominent individuals such as philosophers, political leaders, and elite women, who freely exchanged ideas and engaged in intellectual equality. This community was crucial in the international exchange and transmission of ideas, during an era where literacy rates were climbing, and printed materials were becoming more accessible to a broader audience.