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

Action Reasoning:
To place block 'a' onto block 'c', we first need to move block 'a'. Since it is currently on block 'd', which is on top of a stack, we need to unstack it to free it from the stack.

New State Description:
Block 'a' is now in the hand and is not clear. Block 'd' is on top of block 'b' and clear. Block 'b' is on block 'c' and supporting block 'd'. Block 'c' is on the table supporting block 'b'. The hand is holding block 'a'.

Action Chosen:
unstack(block_a, block_d)
