Control accounting is a technique used to monitor and assess the efficiency of bookkeeping by accountants in recording and reporting all business transactions of an enterprise. It involves setting up control accounts that summarize the balances of all the individual accounts in the ledger and comparing these balances with the actual amounts in the individual accounts. This helps to identify any errors or omissions in the accounting records and allows for prompt corrective action to be taken. Therefore, the correct answer is "monitoring the efficiency of bookkeeping by accountants".