The correct answer is B) The fact that they are the largest surviving collection of 15th Century English correspondence.
These letters are especially important for historians since as the largest collection of 15th century correspondence they provide ample details on all the social, cultural, political, legal, religious and economic mores of Medieval England. Additionally they are important with regards to the history of the English language as they are written in the language of the era which was quite different from Modern English.
