The phase crossover frequency occurs when phase is:

• A. 0°
• B. −90°
• C. −180°
• D. +90°

Answer: (C)

The phase crossover frequency is defined as the frequency at which the phase angle of a system's transfer function is −180°. This particular frequency is critical in control theory and signal processing because it indicates the point at which the system's response can become unstable under feedback conditions.

At −180°, the output of a system is in opposition to the input, which can lead to constructive interference and potential instability if feedback is applied. Understanding this frequency helps in designing control systems to ensure stability and proper response to inputs.

In resonance or feedback systems, when the phase reaches −180°, it means that the input and output signals are perfectly out of phase, which is a significant condition to monitor in stability analysis. The other phase angles provided do not denote the phase crossover frequency, as their values do not lead to the same critical stability consideration that occurs at −180°. Thus, the choice of −180° accurately captures the essence of the phase crossover frequency.