The correct answer is: D. None of the above
A system with gain margin close to unity or a phase margin close to zero is marginally stable. This means that the system is on the verge of instability. A small change in the system’s parameters could cause the system to become unstable.
A system is said to be stable if, after a disturbance, it returns to its original state. A system is said to be unstable if, after a disturbance, it does not return to its original state, but instead continues to oscillate or diverge.
The gain margin is a measure of how much the gain of a system can be increased before the system becomes unstable. The phase margin is a measure of how much the phase of a system can be increased before the system becomes unstable.
A system with gain margin close to unity or a phase margin close to zero has very little margin for error. A small change in the system’s parameters could cause the system to become unstable.