Which aspect of cable and DSL Internet options makes cable more variable in speed?

Cable Internet's variability in speed is primarily due to the sharing of bandwidth among users. In a cable Internet system, multiple households or businesses connect to the same cable node, and they all share the available bandwidth. This means that during peak usage times—when many users in the same area are online simultaneously—the speeds can decrease significantly for everyone. Since the bandwidth is divided among these users, the more people that are using the service at the same time, the less bandwidth there is available for each individual connection, resulting in slower speeds.

In contrast, DSL (Digital Subscriber Line) operates over telephone lines and typically provides a more consistent speed because it is not subject to the same level of shared usage. Each DSL user has a dedicated connection to the internet through the telephone line, which does not fluctuate based on the number of connected users in the same way cable does.

The other options do not accurately capture the reason for cable's variability in speed. The dependence on telephone lines is more indicative of DSL services, while fiber-optic connections generally provide increased speed and reliability. Fixed connection speeds typically imply a consistent and predictable service, which is not the case for cable due to its shared nature.