**Explanation:**

The given statement implies that the system of accounting being referred to is not reliable. Let's examine each option to understand why the system is considered unreliable.

a) **Trading and profit and loss account cannot be prepared:**

The trading and profit and loss account is an important financial statement that summarizes the sales, expenses, and profit or loss of a business during a specific period. It provides valuable insights into the performance and profitability of the business. If the system of accounting cannot generate a trading and profit and loss account, it indicates a significant flaw in the system's ability to accurately track and record financial transactions. Without this statement, it becomes difficult to assess the profitability of the business and make informed decisions.

b) **Balance sheet cannot be prepared:**

The balance sheet is a financial statement that presents the financial position of a business at a specific point in time. It provides a snapshot of the business's assets, liabilities, and owner's equity. The balance sheet is crucial for understanding the financial health and stability of a business. If the system of accounting cannot generate a balance sheet, it indicates a fundamental flaw in the system's ability to accurately record and report financial transactions. Without a balance sheet, it becomes challenging to assess the overall financial position, leverage, and liquidity of the business.

c) **Trial balance cannot be prepared:**

The trial balance is a statement that lists all the general ledger accounts and their respective debit or credit balances. It is prepared to ensure the equality of debits and credits and to identify any errors or discrepancies in the accounting records. The trial balance is an essential step in the accounting process as it forms the basis for preparing the financial statements. If the system of accounting cannot generate a trial balance, it indicates a significant deficiency in the system's ability to maintain accurate and balanced accounting records. Without a trial balance, it becomes difficult to identify and rectify errors, leading to unreliable financial statements.

**In Conclusion:**

The given options a), b), and c) collectively indicate that the system of accounting lacks the necessary functionality to prepare essential financial statements like trading and profit and loss account, balance sheet, and trial balance. These statements are vital for assessing the financial performance, position, and accuracy of the accounting records. Therefore, the system of accounting is considered unreliable due to its inability to generate these critical financial statements.