Answer:
b. debit Bad Debt Expense, $13,600; credit Allowance for Doubtful Accounts, $13,600
Explanation:
Allowance for Doubtful Accounts is a contra asset account, It normally has credit balance but an adjustment may also make it's balance debit.
The uncollectible receivables of $13,000 means the there should be a credit balance in Allowance for Doubtful Accounts at the end of the period. This account already has debit balance of $600 which is also needs to be adjusted.
Total adjustment = $13,000 + $600 = $13,600