Suppose I am certain that I will choose A? (Maybe an oracle I completely trust has told me, or maybe I have in the past always chosen A; I am not saying anything about the certainty being justified; cf. this paper by David Hunt.) Can I deliberate between A and B, and choose A?
Here is an argument for a positive answer. Suppose I am 99% sure I will choose A. Clearly, I can deliberate between A and B, and freely choose either one (assuming none of the other reasons why I might be unfree apply). The same is true if I am 99.9% sure. And 99.99%. And so on. Moreover, while in such cases it may (though I am not sure even of that) become psychologically harder and harder to choose B, except in exceptional cases (see next paragraph), it should not become psychologically any harder to choose A over B. But if it does not become any harder to choose A over B, why can't I still deliberate and choose A in the limiting case where the certainty is complete?
There will be special cases where this limiting case argument fails. These will be cases where either I suffer from some contingent psychological condition that precludes a choice in the case of certainty or where in the limiting case I lose the reason I had for doing A. For instance, if I am in weird circumstances, there may be actions where the reasons I have for choosing the action depend on my not being certain that I will choose it—maybe you offer me money to choose something that I am not certain I will choose. But apart from such special cases, the probability that I will choose A is irrelevant to my deliberation. And hence it does not enter into my deliberation if I am being rational.
What enters into my deliberation whether to choose A or B are the reasons for and against choosing the options. The probabilities or even certainties of my making one or the other choice normally do not enter into deliberation.
But what if I know that it is impossible to do B? Isn't that relevant? Normally, yes, but that's because it's a straightforward reason against choosing B: one has good reason not to choose to do things which one can't succeed in doing, since in choosing such things, one will be trying and failing, and that is typically an unhappy situation.
But what if I am certain that it is impossible to choose B? Isn't that a reason against choosing B? I am not sure. After all, it might be a reason in favor of choosing B—wouldn't it be really cool to do something impossible? Maybe, though, what the case of impossibility of choice does is it removes all the reasons in favor of choosing B, since reasons involve estimates of expected outcomes, and one cannot estimate the expected outcome of an impossibility, perhaps.