The correct option is C subsidiary book as well as principal book
Cash book is a book in which all transactions relating to cash receipts and cash payments are recorded. It starts with the cash or bank balances at the beginning of the period. Generally, it is made on monthly basis. This is a very popular book and is maintained by all organizations, big or small, profit or not-for-profit. It serves the purpose of both journals as well as ledger (cash) account. It is a subsidiary book and also called a book of original entry/principal book. When a cash book is maintained, transactions of cash are not recorded in the journal, and no separate account for cash or bank is required in the ledger.