Answer: A. The increase in the PBO attributable to applying the more generous terms of the amendment to prior service years is the prior service cost
B. The prior service cost is the difference between the PBO at the end of the previous year that was calculated without regard to the amendment and the recalculated PBO based on the plan amendment
Step-by-step explanation:
Both options A and Be are correct because as the question says, the PBO can change due to the accumulation of service cost from year to year. This means that the increase in the PBO attributable to applying the more generous terms of the amendment to prior service years is the prior service cost.
This is also the reason why we can calculate the prior service cost by subtracting from the previous PBO that was not yet amended, the new PBO that is amended because the service cost is what the PBO increased by.