Which of the following is most likely to be judged as a significant residual risk...

A significant residual risk that would exceed the organization's acceptable risk level is likely to be one that has severe consequences, such as causing injuries or environmental pollution. These types of risks can have substantial legal, financial, and reputational impacts on an organization and are typically beyond acceptable levels of risk tolerance. References:

COSO’s Enterprise Risk Management – Integrating with Strategy and Performance.

The IIA’s Practice Guide on Risk Management.