The correct answer is: Error of commission.
An error of commission is an error that occurs when a transaction is recorded incorrectly. In this case, the sale of Rs. 30,000 to A was entered as a sale to B. This is an error of commission because the transaction was recorded incorrectly, even though the total amount of the sale was correct.
An error of omission is an error that occurs when a transaction is not recorded at all. In this case, if the sale of Rs. 30,000 to A had not been recorded at all, it would have been an error of omission.
A compensating error is an error that occurs when two errors cancel each other out. In this case, if the sale of Rs. 30,000 to A had been entered as a sale to B, but the sale of Rs. 30,000 to B had been entered as a sale to A, it would have been a compensating error.
An error of principle is an error that occurs when a transaction is recorded incorrectly because of a misunderstanding of the accounting principles involved. In this case, if the sale of Rs. 30,000 to A had been entered as a sale to B because the accountant did not understand that the sale should be recorded to A, it would have been an error of principle.