Final answer:
A spectrum analyzer is the best device for diagnosing cable telephony or high-speed data cable modem performance issues as it can analyze signal interference and strength. Therefore the correct option is B) Spectrum Analyzer
Step-by-step explanation:
When trying to isolate the cause of cable telephony or high-speed data cable modem performance problems, the best choice among the options provided is a B spectrum analyzer.
A spectrum analyzer is a device used to examine the signal spectrum of electrical signals. It helps in detecting and analyzing issues such as signal interference, bandwidth problems, and signal strength which are crucial for diagnosing problems related to cable modem performance.
Other options such as a signal splitter or a cable modem itself won't be able to provide a detailed analysis of signal problems.
An optical node is part of the cable network infrastructure, and while it is important for overall network performance, it doesn't serve as a diagnostic tool for specific issues with cable modem performance.
Therefore the correct option is B) Spectrum Analyzer