SAT Question e8926747: Form Structure And Sense answer and explanation

English - Form Structure And Sense - Pronoun Antecedent Agreement
0:00
FREE OFFICIAL BANK
Eighteen letters written by Louisa May Alcott, author of the popular novel Little Women (1868), can be found at the New York Historical Society. ______ letters demonstrate Alcott’s keen business sense in her interactions with publishers.
187
Difficulty:Easy
Which choice completes the text so that it conforms to the conventions of Standard English?