Attempt Number: 4
Error Message: Action not unique compared to other actions chosen from this state before. Choose a new action

Action Reasoning:
To proceed towards the goal state, we need to ensure block `b` is clear so we can stack `d` on it. Therefore, I'll first unstack block `c` from `b`.

New State Description:
Block `a` is on the table and clear. Block `b` is on the table and clear, as block `c` has been removed. Block `c` is now held in the hand and no longer on `b`. Block `d` is on the table and clear. The hand is holding block `c`.

Action Chosen:
unstack(c, b)
