a) The best action you should be doing now is to sleep or at least to take a nap, so your brain would process the information it has now and prepare itself for new information

I don't think this is how Kurzban's model would explain it. In Kurzban's model, the "feeling of exhaustion" stems from one or more monitoring mechanisms causing aversive states (in response to trying to decide what to do). The monitoring mechanisms aren't causing the feeling of exhaustion so that you'll feel sleepy - they are just voting by causing aversive states, and Kaj_Sotala happens to interpret that feeling of the aversive states with the word "exhaustion" (which has certain unhelpful connotations and I think should be taboo'd in favor of "aversive state feelings").

c) Your brain is wrong. For reasons that might make sense living in a jungle millenia ago, your brain is convinced that you should do X, which in reality is not an option, or is a bad option. All meaningful options are penalized because your brain insists that you should do X instead. For example, if your job made you too exhausted, your brain may think the best action is to prevent going to the same job tomorrow. Which may be not realistic, or merely not what you want to think about.

I also don't think this applies the model correctly. The feeling of exhaustion would need to stem from aversive states from monitoring mechanisms that are working at the moment you get back from work and are deciding what to do next. Returning to work the next day is a very distant action, relative to all the activities your brain would be thinking about doing at that moment.

b) You are censoring the best action from your thoughts because it does not fit your idea of what you should be doing now. For example you feel that you should work on your personal projects, but your brain thinks you should drink some beer with your friends. When you say you are too exhausted to do anything, you are actually lying, because you are not too exhausted to drink beer with your friends; it is merely outside of your set of acceptable answers.

I think this is more how Kurzban's model would explain this situation. The bad feeling (which Kaj calls "exhaustion") would need to come from multiple monitoring mechanisms returning aversive states for evaluating various possible next actions (in this case, maybe working on projects, watching tv, browsing the internet, practicing, or studying).

It could be that there is no strong winner (so Kaj ends up feeling just the aversive states but not a clear idea of what to do next and has to spend more time feeling that exhaustion before a clear winner emerges). In this model, I think it would be possible for no strong winner to emerge even when there are immediately gratifying activities due to some monitoring mechanisms paying attention to some sort of executive function-centered value system (like "I should feel bad for doing <instantly gratifying activity>"). If that were the case, I would think Kaj might feel a sort of "oscillation" between deciding to do something instantly gratifying (which is generally a quick judgement for a brain to make) but then deciding not to after the executive function/value system evaluation results are returned (which I think generally takes longer to get a result from).

It could also be that Kaj's brain did actually reach a conclusion on what to do next, but either wasn't introspective enough to figure out what that was and put it into words or they meant the phrase "not doing anything at all" to be a catch-all for some unproductive but instantly gratifying action.

W.r.t. this seeming to occur after "a long day at work", I'm not really clear what is meant by "a long day".