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

Action Reasoning:
To move towards the goal of stacking block 'a' on block 'c', I will use the stack action to place block 'a' on block 'c'.

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

Action Chosen:
stack(block_a, block_c)
