Answer:
b.The adjustment was posted as a debit to Accounts Receivable for $870 and a credit to Fees Earned for $780
Explanation:
Event though those accounts are the ones, and it´s right to debit and credit in that way, the adjustment must balance.
In each journal entry, sum of Debits must be equal to sum of Credits, otherwise TRIAL BALANCE WILL BE WRONG.
In this case, there is a difference of $ 90 ( DEBIT > CREDIT )