What is included in the formula to calculate the ending Projected Benefit Obligation (PBO)?

To determine the ending Projected Benefit Obligation (PBO), it is essential to incorporate various components that reflect changes in the obligation over the period. The correct answer correctly includes the beginning PBO, which serves as the starting point for calculating any changes during the period.

Adding service cost accounts for the increase in the PBO due to the additional benefits earned by employees during the current period. Interest cost also contributes to the obligation because it reflects the time value of money; as time passes, the obligation increases due to the accumulation of interest. The mention of actuarial gains is crucial as these represent the adjustments based on changes in assumptions or estimates that lead to a decrease in the PBO. Therefore, they are subtracted from the total to reflect a more accurate ending PBO.

This approach takes a comprehensive view of how obligations evolve through service costs, interest, and adjustments for actuarial calculations, providing a complete picture of the ending obligation's calculation. Thus, option B captures all necessary components to accurately reach the ending PBO, illustrating the multifaceted nature of pension obligation calculations.