"If God knows I will not choose the red shirt" is a conditional statement.If God knows I will not choose the red shirt, how is the red shirt an available option to me?
You are the one who is suggesting it ("if"), so you cannot make any conclusion about
"what is available to choose".
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If God knows I will not choose the red shirt, I will choose the blue shirt.
If God knows I will not choose the blue shirt, I will choose the red shirt.
The above shows clearly that it could be either of the two options.
i.e. Both options are available.
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or another way of understanding it:
"If God knows I will not choose the red shirt" does not mean that I cannot choose
the red shirt, because if I will choose the red shirt, then the original conditional
is incorrect ("If God knows I will not choose the red shirt" is false)