Choice "C" is correct. A task (a box on a document or system flowchart) to compare control totals and remittance totals would be included in the processing of cash receipts. Note that this is a very general step that could appear on almost any document flowchart. All of the other choices in this question are specific to a particular type of processing.
Choice "a" is incorrect. Recording returns and allowances would not be included in the processing of cash receipts. Anything returned will not be received in cash. Returns and allowances would be included in the processing (and thus the document flowchart) of sales.
Choice "d" is incorrect. Authorizing and generating invoices would not be included in the processing of cash receipts. Invoices would be included in the processing of sales.
Choice "b" is incorrect. Authorizing and generating vouchers would not be included in the processing of cash receipts. Vouchers would be included in the processing of accounts payable and payments.