To calculate the interest due, we first need to determine the average daily balance for the month.
From the beginning of the month until the 9th day, the balance is $660. From the 10th to the 27th, the balance is $660 - $90 = $570. On the 28th, the balance increases to $570 + $320 = $890.
So the average daily balance is:
[(31 - 9) x $570 + 9 x $660 + 22 x $890] / 31 = $691.29
Next, we need to calculate the monthly interest rate:
26% APR = 0.26 / 12 = 0.02167
Finally, we can calculate the interest due:
$691.29 x 0.02167 = $14.98
Therefore, the answer is closest to option (a) $7.48.