The answer should be b) Interest Income and Interest Expense.
A pair of accounts generally represents a transaction, so in answer choice a), you debit "Unearned Revenue" and credit "Service Revenue", because you are trying to show that because you performed a service/delivered a good, you are now able to recognize revenue, which reduces unearned revenue.
c) and d) are both possible too, because your balance in prepaid rent is decreasing by your rent expense, and by incurring salaries expense, you owe your employees a salaries payable.
b) is not possible because there is no transaction whereby interest income can cause interest expense, or vice versa.
