An oscilloscope is primarily used to:

An oscilloscope is primarily used to visualize and analyze electrical waveforms, making it an essential tool in electrical engineering and electronics. It provides a graphical representation of voltages over time, allowing users to see the waveform's shape, amplitude, frequency, and any changes occurring in the signal. This capability is crucial for diagnosing problems in circuits, verifying the performance of electronic components, and understanding the behavior of signals in various applications.

The oscilloscope does this by using probes to capture electrical signals, converting these signals into a visual display on the screen. This feature is invaluable for engineers and technicians as it enables them to monitor dynamic signals in real-time, facilitating detailed analysis and troubleshooting.

The other choices refer to functions of different tools or concepts. Measuring resistance in a circuit is typically done with a multimeter, while storing electrical energy relates to capacitors or batteries. Testing insulation quality would involve using an insulation resistance tester or a similar device, not an oscilloscope. Therefore, the correct answer reflecting the primary function of an oscilloscope is its ability to visualize and analyze electrical waveforms.