Answer:
b. Cash 500 Accounts Receivable 500
Explanation:
This entry is needed for the settlement of account-based services offered to customers. As a result, we credit Accounts Receivable by $500 for payments obtained from customers that were already billed for those services.
So, Journal entry will be as follows,
    Cash A/c Dr.                    500
To,  Accounts Receivable A/c              500
Hence, option b is the correct answer.