Which penalty is calculated as a percentage of unpaid taxes for each month it's overdue?

The late payment penalty is the correct choice because it is specifically assessed as a percentage of the unpaid tax amount for each month or part of a month that the tax remains unpaid after the due date. This penalty is designed to encourage taxpayers to pay any taxes owed in a timely manner, thus minimizing delays in tax revenue collection.

When a taxpayer fails to pay their owed taxes by the due date, the late payment penalty accrues monthly, allowing the tax authority to impose a financial consequence that grows with each passing month. This structure serves both to deter late payments and to compensate the government for the time value of money lost due to the taxpayer's delay in settling their obligations.

Other penalties, such as the interest penalty, are typically charged based on the amount of unpaid tax but might differ in calculation methods and purposes, focusing on the interest cost rather than a straightforward percentage of the overdue balance. The late filing penalty, on the other hand, is related to the timeliness of filing the tax return rather than the payment of the tax itself, and the failure to pay penalty is often considered synonymous with the late payment penalty. Thus, the late payment penalty uniquely aligns with the description provided in the question.