Attempt Number: 4
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 place 'a' on top of 'b'. Currently, 'b' is not clear, so we should first unstack 'd' from 'c' to clear 'c', which will then clear 'b' as well.

New State Description:
Block 'a' is on the table and clear. Block 'c' is on top of block 'b' and clear. Block 'b' is on the table. Block 'd' is held in the hand and is no longer on top of block 'c'. The hand is not empty.

Action Chosen:
unstack(d, c)
