Attempt Number: 3
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, block 'a' should be placed on block 'c'. Currently, the hand is holding block 'a' and block 'c' is clear and on the table, so the next logical step is to stack 'a' on 'c'.

New State Description:
Block 'a' is now on block 'c', which is on the table. The hand is empty. Block 'c' is supporting block 'a' and is not clear. Block 'b' is supporting block 'd' and is not clear. Block 'd' is on block 'b' and is clear.

Action Chosen:
stack(a, c)
