Attempt Number: 4
Error Message: Action not unique compared to other actions chosen from this state before. Choose a new action

Action Reasoning:
To progress towards the goal state, we should move block 'a' to be on top of block 'b'. Since block 'a' is clear and the hand is empty, the correct action is to unstack 'a' from 'c'.

New State Description:
Block 'a' is held in the hand and is no longer on block 'c'. Block 'c' is now clear and on top of block 'b', which is on the table. Block 'd' remains on the table and clear.

Action Chosen:
unstack(a, c)
