Past service cost, as defined by accounting standards, refers to the increase or decrease in the present value of the defined benefit plan obligation resulting from a modification to the plan that relates to the service provided by employees in prior periods. In the context of Bennet Co., if there was an amendment to the defined benefit plan during the year, and the amendment resulted in a change to the obligation, then the change in the obligation specifically due to the plan amendment would be considered a past service cost.

Therefore, the correct answer is:

D. The change in the obligation for plan amendment and the turnover percentage

This is because the change in the obligation due to the plan amendment is directly related to a modification of the plan, which falls under the category of past service cost. The turnover percentage, although it affects the overall obligation, does not represent a direct cost associated with a change to the plan and thus is not classified as a past service cost according to the definition.