Hi Deepti,
I agree with you that the risk register would have been a more appropriate option, particularly given the explanation provided. However, keep in mind that in the PMP exam you will have to pick the best (or the least worse, if you will) answer given the options provided. Based on that, I still find option 3 as better than the others.
I tend to look at a mitigation plan as one of the possible risk response strategies, which assumes that the risk has been identified and you are planning a response in case it occurs. Instead, the scenario here clearly mentions that you are in the final stages of the execution, and something occurs. Assume the risk was not previously identified, now that it has occurred it is too late to come up with a mitigation plan.
Hope this helps,
Gabriella