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, I need to first clear block `b` by removing block `c`. Then, stack block `d` onto `b`. So, I will first unstack `c` from `b` to proceed toward this order.

New State Description:
Block `a` is on the table and clear. Block `b` is on the table and clear. Block `c` is in the hand. Block `d` is on the table and clear. The hand is not empty, holding block `c`.

Action Chosen:
unstack(c, b)
