The best version of the underlined portion of sentence 15 is the use of a semicolon after 'holidays' as it correctly separates closely related independent clauses. So, the correct option is D.
You've asked which is the best version of the underlined portion of sentence 15 concerning the use of punctuation after the word 'holidays.' The best punctuation in this context is a semicolon.
A semicolon is used to join two independent clauses that are closely related, and in this sentence, 'even during holidays' is closely related to the clauses that follow it but can stand alone as a complete thought if necessary. Therefore, option D 'holidays;' would be the best choice to convey that the Book Thing will be open every Saturday and Sunday regardless of holidays, World War III, or the apocalypse.