The correct choice is (a) |z| < 1
Explanation: We know that, for the system to be stable, the ROC should include the unit circle.
|z| = 1, represents the unit circle but does not include it.
|z| > 1, represents the region outside the unit circle. In other words, it excludes the region of the unit circle.
|z| ≠ 1 does not represent a unit circle.