Attempt Number: 3
Error Message: Action not unique compared to other actions chosen from this state before. Choose a new action

Action Reasoning:
We need block `b` to be clear so that we can stack block `d` on it, taking us closer to the goal state.

New State Description:
Block `c` is in the hand and no longer on block `b`. Block `b` is on the table and clear, ready to receive block `d`. Block `a` remains on block `d` and clear. The hand is holding block `c`.

Action Chosen:
unstack(c, b)
