Asklemmy

The brain as far as we know works like nowadays "ai" it makes assumptions and tries to get the best and most fitting outcome for itself, this decision is influenced by a astronomical amount of data (thats the difference to nowadays ai) and (for the lack of better scientific measurements) emotions, meaning that theoretically it is possible to perfectly predict your choices, however, if you know about said prediction it will influence the system again.

Saying you have to pick between a Red and a Green Gummy Bear with the exact same taste, you will probably pick the red one because the subconscious associates red usually with food (red and yellow, that's btw why many fast food chains use those colors) however, if you are aware of this, this will likely end up in picking the green one as rebellious act against nature, wich again is predictable because humans are self aware and don't want "to be slave to their own instincts" now if you know about the prediction, the only way to "be unpredictable" would be to not chose a Gummy Bear thus "breaking" the test... Wich is predictable. This is of course a very simplistic experiment but gets the point across, you can't be unpredictable unless you break the laws of physics. This does however not prove or disprove the "free will" inside the system you are in, you could chose things freely, you just don't because its unnecessary and time consuming.

TLDR: we don't know.