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

Action Reasoning:
The next best action is to stack block 'a' on block 'd' because it directly moves us closer to the goal state by placing 'a' on 'd', as required.

New State Description:
Block 'a' is now on top of block 'd', and block 'd' is no longer clear. The hand is empty. Blocks 'b' and 'c' remain clear and on the table.

Action Chosen:
stack(a, d)
