Introduction:
A residential telephone line is a connection that allows individuals to make and receive phone calls from their homes. It typically consists of several components that work together to provide reliable communication services.

Constituents of a Residential Telephone Line:
1. A high-speed downstream channel:
- This refers to the data transmission rate from the service provider to the user.
- It enables fast internet access and allows for quick downloading and streaming.
- This channel is an essential component of modern residential telephone lines, often referred to as broadband connections.

2. A medium-speed downstream channel:
- This channel provides a lower data transmission rate compared to the high-speed downstream channel.
- It allows for basic internet browsing, email access, and other online activities with moderate data requirements.
- While not as fast as the high-speed channel, it still provides a satisfactory user experience for most residential users.

3. A low-speed downstream channel:
- This channel has the lowest data transmission rate among the three options.
- It is typically used for basic telephone services, such as voice calls, without any internet connectivity.
- While it may not support data-intensive activities, it is sufficient for traditional telephone communication.

Which is not a constituent?
The correct answer is option C, a low-speed downstream channel. This is because a low-speed downstream channel is indeed a constituent of a residential telephone line. It is used for traditional voice calls and is part of the overall telephone service.

Conclusion:
A residential telephone line consists of various components to provide voice and data services to users. These include high-speed downstream channels for fast internet access, medium-speed downstream channels for basic internet usage, and low-speed downstream channels for traditional voice calls. Each channel serves a specific purpose and contributes to the overall functionality of the residential telephone line.