Choice "D" is correct. Preparing invoices and recording the related receivables are both recordkeeping functions that would not be inconsistent with each other.
Choice "c" is incorrect. This would allow the clerk to misappropriate cash while still reducing the receivable, such as in a lapping scheme.
Choice "a" is incorrect. The purpose of a bank reconciliation is to uncover unexplained discrepancies between the bank balance and the cash on hand. If the reconciliation is not performed by someone independent, the cashier may be able to misappropriate cash and conceal the theft.
Choice "b" is incorrect. This would allow the purchasing manager to purchase and pay for unauthorized goods with little oversight from others.