What is the correct form of the verb to complete the sentence: The books on the shelf ___ arranged by author's name in alphabetical order?

The correct form of the verb in the sentence is "are," which effectively agrees with the plural subject "books." In English grammar, the verb must match the subject in number; since "books" is plural, a plural verb form like "are" is required. Using "are" indicates that the action of arrangement is currently applicable to the multiple books on the shelf. This alignment ensures clarity and grammatical correctness in the sentence, conveying that it is the books that are organized in the specified manner.