Attempt Number: 2
Error Message: Action not unique compared to other actions chosen from this state before. Choose a new action

Action Reasoning:
Since block `b` must be on the table and block `d` must be on block `b` in the goal state, placing block `d` on block `b` is necessary. The hand is currently holding block `d`, block `b` is clear on the table, making stacking `d` on `b` the logical next step.

New State Description:
Block `a` is on the table and clear. Block `b` is on the table and not clear. Block `c` is on the table and clear. Block `d` is on top of block `b` and clear. The hand is empty.

Action Chosen:
stack(d, b)
