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

Action Reasoning:
To achieve the goal state, we need to reverse the positions of blocks B and C. The first step is to unstack block B from block C, which is already clear and satisfies the preconditions.

New State Description:
Block c is on the table and clear. Block b is in the hand and no longer on block c. Block a is on the table and clear. Block d is on the table and clear. The hand is holding block b.

Action Chosen:
unstack(b, c)
