While preparing your risk responses, you realize that you have not planned for unknown risk...

Include a management reserve in the budget to try to compensate for the unknown risks, and notify management to expect unknown risks to occur.

Document the unknown risk items and calculate the expected monetary value based on probability and impact that may result from the occurrence.

Determine the unknown risk events and the associated cost, then add the cost to the project budget as reserves.

With the stakeholders, determine a percentage of the known risk budget to allocate to a management reserve budget.