Which of the following is principal book of accounting system?

The correct answer is option 2- Ledger.

The ledger is the principal book of accounting system. It contains different accounts where transactions relating to that account are recorded. A ledger is the collection of all the accounts, debited or credited, in the journal proper and various special journal. A ledger may be in the form of bound register, or cards, or separate sheets may be maintained in a loose leaf binder. In the ledger, each account is opened preferably on separate page or card.

OTHER OPTIONS-

Journal- Journal is the basic book of original entry. In this book, transactions are recorded in the chronological order, as and when they take place.

Cash book- Cash book is maintained to record all the cash transactions.

Sales book- It records all the credit sales.