Attempt Number: 3
Error Message: Action not unique compared to other actions chosen from this state before. Choose a new action

Action Reasoning:
Block 'a' is currently being held and block 'c' is clear. To progress towards the goal state, stack 'a' on 'c' so that both blocks are positioned correctly for the final arrangement.

New State Description:
Block 'a' is stacked on block 'c', which is on the table. The hand is now empty. Block 'c' is no longer clear, but block 'a' is clear. Block 'b' remains on the table with block 'd' on top of it, and block 'd' is clear.

Action Chosen:
stack(a, c)
