What is the common characteristic impedance for voice-grade transmission lines?

The common characteristic impedance for voice-grade transmission lines is 600 ohms. However, considering the options listed, 75 ohm is often associated with video and broadcast applications, not with traditional voice-grade services.

Meanwhile, the standard for telecom applications, including voice, typically utilizes a characteristic impedance of 100 ohms for balanced twisted-pair cabling, like that used in structured cabling systems for data communication. This impedance reflects the typical requirements of network infrastructure for digital and analog voice signals.

In the context of the options provided, it's essential to assess the commonly used impedance for voice-grade systems. Twisted-pair cabling systems, which are prevalent in voice communication setups, follow the 100-ohm standard, making it the most appropriate choice here.