No physical entity or contrasting ‘item’ at all, in fact. The tree itself bore fruit of no special significance whatsoever, just an ordinary tree. Much like the others.
Except for perhaps only one thing. The fact that our Father had named it as standing different from the rest. A test, with a foreseen outcome.
Free will and choice, knowledge of good and evil, were a prerequisite state prior to and along side the event of the tree. So the physical ‘tree’ is almost irrelevant to the experience gained by A&E. Yet at the same time a key component of it.
The Tree represents a test but also a choice simultaneously.
You might say the consequence of the choice depended on the tree. So it was a prop, for a God who already knows that we all have much to learn. An unfathomable volume infact.