Attempt Number: 2
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 stack 'b' on 'a'. Since 'b' is clear and on the table, and the hand is empty, we can pick up 'b' to prepare for stacking.

New State Description:
The hand is holding block 'b'. Block 'a' is on block 'c', making block 'a' not clear. Block 'd' remains on the table and clear. Block 'c' is supporting block 'a' and is not clear.

Action Chosen:
pick-up(b)
