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, the topmost block, `block_d`, needs to be moved to `block_b`. The first necessary step is to clear `block_d` from its current stack, which requires taking `block_d` off `block_a`.

New State Description:
Block `b` is on the table, supporting `block_c`. Block `c` is on `block_b`, supporting `block_a`. Block `a` is on `block_c`, clear, since `block_d` was removed. Block `d` is held by the hand. The hand is not empty.

Action Chosen:
unstack(d a)
