Choice "D" is correct. Confirmation responses received electronically should be verified by calling the sender. It is possible for such responses to look like they came from the respondent when in fact they were actually generated by another party, such as the client.
Choice "a" is incorrect. Confirmations are not required if the balance of receivables is immaterial.
Choice "b" is incorrect. Verification of subsequent cash receipts provides excellent evidence about year-end receivables, and would be an appropriate alternative procedure when responses are not received.
Choice "c" is incorrect. Negative confirmations only request a response when the amount stated is incorrect, so there would be no need to perform additional procedures in this case.