Risk mitigation is most effective when the residual risk is optimized, as it means that the risk exposure and impact have been reduced to the level that is aligned with the risk tolerance and appetite of the organization, and that the risk response is cost-effective and optimal. The other options are not the factors that determine the effectiveness of risk mitigation, as they are more related to the types or sources of risk, respectively, rather than the level or outcome of risk. References = CRISC Review Manual, 7th Edition, page 111.