Explanation: A Time-Domain Reflectometer (TDR) operates by sending a very short, fast electrical pulse down a transmission line. Any discontinuities in the line (such as shorts, opens, or impedance mismatches) will reflect a portion of this pulse back towards the source. An **oscilloscope** is essential for a TDR because it graphically displays voltage changes over time. It allows you to see the original outgoing pulse and any reflected pulses. The time delay between sending the pulse and receiving its reflection indicates the distance to the discontinuity, and the shape of the reflection reveals its nature (e.g., open circuit, short circuit, impedance change). A **pulse generator** provides the necessary short, high-speed electrical pulses that are injected into the transmission line. Therefore, an oscilloscope and a pulse generator are the fundamental components combined to create a TDR for analyzing transmission lines. * **A) Frequency spectrum analyzer, RF generator:** This combination is used for frequency domain analysis, not time-domain reflection. * **C) AC millivolt meter, AF generator:** These are used for audio frequency measurements and basic voltage checks, not for high-speed pulse analysis. * **D) Frequency counter, linear detector:** This combination measures signal frequency and amplitude, but not the time-based reflections crucial for TDR.