Answer: Increase in the projected benefit obligation due to the passage of time.
Step-by-step explanation:
The interest cost is used to show how much the Projected Benefit Obligation (PBO) owed by the employer to the Defined Benefit pension plan has increased by in the period.
The interest cost increases based on the unpaid beginning PBO for the period that arises as the employment term of the employee increases and the interest rate is the discount rate that is used to discount the PBO.