Go through the prepcast questions and read it carefully. PMP exam questions are very similar to PREPCAST. Reviewing only prepcast paper should be fine.
The question states that this was an unidentified risk from planning.
You are most likely in project execution and you just uncover a risk that was not planned. There is no risk response plan, therefore you need to create a workaround to mitigate the risk. To create the workaround, the best course of action would be to brainstorm with the team.
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