The correct answer is D. Assets books.
A subsidiary book is a book of original entry that is used to record specific types of transactions. Purchase book, sales book, and bills receivables book are all subsidiary books. Assets books, on the other hand, are not subsidiary books. They are general ledger accounts that are used to record all assets, regardless of the type of transaction.
A purchase book is used to record all purchases of goods and services. A sales book is used to record all sales of goods and services. A bills receivables book is used to record all bills receivable, which are amounts that customers owe to the business. Assets books, on the other hand, are used to record all assets, regardless of the type of transaction. Some examples of assets include cash, accounts receivable, inventory, property, plant, and equipment.
Subsidiary books are important because they provide detailed information about specific types of transactions. This information can be used to track inventory levels, monitor accounts receivable, and analyze sales trends. Assets books are also important, but they do not provide the same level of detail as subsidiary books.