The completed service increases Service Revenue and Accounts Receivable. The later collection increases Cash and decreases Accounts Receivable.
The completed and accepted service gives the company a receivable; the invoice documents that amount.
Cash collection increases Cash and decreases Accounts Receivable. It does not increase revenue again.
Recording revenue again at collection would count the same service twice.
Answer: A
Harbor Research records $8,400 of Accounts Receivable and Service Revenue on September 28. On October 18, it records Cash and removes the receivable. The invoice documents the amount but does not cause another accounting entry.