The correct answer is: A. low frequencies.
A conditionally stable system is a system that is stable for some values of the open-loop gain but unstable for other values. The system exhibits poor stability at low frequencies because the phase margin is small at these frequencies. The phase margin is a measure of how close the system is to instability. A small phase margin indicates that the system is close to instability and that a small change in the open-loop gain could cause the system to become unstable.
Option B is incorrect because reduced values of open-loop gain can actually improve the stability of a conditionally stable system. This is because reduced values of open-loop gain increase the phase margin and make the system less likely to become unstable.
Option C is incorrect because increased values of open-loop gain can actually worsen the stability of a conditionally stable system. This is because increased values of open-loop gain decrease the phase margin and make the system more likely to become unstable.
Option D is incorrect because a conditionally stable system can exhibit poor stability at low frequencies.