All-pass filters are used in various applications, but one of the significant areas where they find application is in telephony.

Telephony:
- In telephony, all-pass filters are used for various purposes, such as echo cancellation and phase equalization.
- Echo cancellation: When a voice call is made over a telephone line, the voice signal can get reflected back due to impedance mismatches or other reasons. This reflected signal is known as an echo. All-pass filters are used in echo cancellation systems to remove or reduce this echo, thus improving the quality of the voice call.
- Phase equalization: In telephony, it is essential to maintain the phase coherence of the transmitted signal. All-pass filters are used to equalize the phase response of the audio signals, ensuring that the phase characteristics of the transmitted signals are preserved.

Other applications:
- Although the correct answer is option 'C', it is worth mentioning that all-pass filters are also used in other applications.
- Cathode ray oscilloscope (CRO): All-pass filters can be used in CROs for phase compensation or to adjust the phase response of the signal being displayed on the screen.
- Television: All-pass filters can be used in television systems for various purposes, such as color correction, phase equalization, or to compensate for the frequency response of the audio signals.
- However, telephony is the prime area where all-pass filters are extensively used.

Conclusion:
All-pass filters find applications in various fields, but in the given options, telephony is the correct application area for all-pass filters. They are used for echo cancellation and phase equalization, ensuring better voice call quality and maintaining phase coherence in telephony systems.