Explanation: An SWR (Standing Wave Ratio) meter is the primary instrument used to verify proper antenna functioning. It measures the impedance match between the radio, transmission line, and antenna. A low SWR (ideally 1:1) indicates that the antenna is resonant and efficiently accepting power from the radio to radiate into the air, which is the definition of proper functioning. A high SWR means a significant portion of the power is reflected back, indicating a mismatch and poor antenna performance. A digital ohmmeter (A) measures DC resistance, which doesn't reflect the antenna's impedance at RF frequencies or its radiation efficiency. A frequency meter (B) verifies the radio's output frequency, not the antenna's performance. Using a different radio (D) might confirm basic operation, but provides no quantitative data on the antenna's tuning or efficiency.