As per PMBOK -
Risk Acceptance is a risk response strategy whereby project team decides to acknowledge the risk and not take any action unless the risk occurs.
It also mentions that The most common active risk acceptance strategy is contingency reserves. So just keep some time, cost or resources buffer to cover for the known risk with unknown response.
If you have 'Contingency Reserves' as option specifically mentioned, then go with that option, rather than a generic 'Risk Acceptance' option.
But I have my doubts about this. How can one estimate the reserves required, if they don't know what has to be done if the risk arises? I mean, how would one estimate that buffer is sufficient to handle the risk contingency or not? Or is it just a certain percentage of time, cost and effort kept aside, and decide on what to do with them when the risk rises? Of course, there will certainly be some ground rules about what percentage can be kept as buffer, but still....