Final answer:
The accumulated postretirement benefit obligation (APBO) at December 31, 2016 is $85 million.
Step-by-step explanation:
The accumulated postretirement benefit obligation (APBO) at December 31, 2016 can be calculated by considering the changes in the obligation during the year. The APBO at the beginning of the year was $75 million. The retiree benefits paid during the year were $9 million. The service cost for the year was $21 million. The actuary revised the estimate of the APBO downward by $2 million.
To calculate the APBO at the end of the year, we need to subtract the benefits paid and add the service cost and the downward revision in the estimate. Therefore, the APBO at December 31, 2016 is: $75 million - $9 million + $21 million - $2 million = $85 million.