Unsettled - Introductory
This makes one large eye, not enough for white to live.
If white doesn't want to play inside, black can play inside. White can capture the two inside stones:
This shape is dead. It's discussed in Dead - Introductory.
So we come to the conclusion that this shape is dead after black plays here first. But what about when white plays?
For the same reason, the bent three is also unsettled: The vital point is at a. Try to see why.
There are other candidates for the simplest unsettled shape: