Answer:
b.$487,900.
Step-by-step explanation:
Underapplied overhead occurs when the actual amount of manufacturing overhead incurred is higher than expected.
Since the manufacturing overhead for the year was underapplied by $23,540 in the question, the estimated manufacturing overhead at the beginning of the year used in the predetermined overhead can be calculated just deducting $23,540 form the actual as follows:
Estimated manufacturing overhead at the beginning of the year = $511,440 - $23,540 = $487,900.