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

Action Reasoning:
The next best action is to put block c on the table to free the hand, allowing us to pick up block d and stack it on block c, moving us closer to the goal state.

New State Description:
The hand is empty. Block a is on the table. Block b is on top of block a and is clear. Block c is on the table and clear. Block d is on the table and clear.

Action Chosen:
put-down(c)
