Choice "A" is correct. The function of cash receipts is part of the treasurer's department and should be separate from the role of posting credits to the A/R ledger. Failure to separate the recordkeeping function from the custodial function allows an individual to misappropriate cash and then cover up the theft by posting credits against the related A/R balance.
Choice "d" is incorrect. If fictitious transactions in the revenue cycle are recorded, then the impact on revenues and receivables would be the same; either both would be overstated (the most likely case) or both would be understated.
Choice "c" is incorrect. Even the lack of effective internal controls would not allow this fraud to be perpetrated for long, since the customers that submitted the claim would complain that the credit was not properly applied as soon as they received their next statement or invoice.
Choice "b" is incorrect. The failure to prepare shipping documents may cause inventory to be overstated, but it is unlikely to be perpetrated as a fraud since it does not allow theft of cash. In addition, the internal controls in the revenue cycle typically relate to sales, receivables, and cash, not to inventory.