What tool can be used to verify the calibration of a leakage detector?

A variable attenuator is the correct tool to verify the calibration of a leakage detector because it allows the user to systematically adjust the signal level that the leakage detector receives. By using a variable attenuator, one can create known signal levels and evaluate how accurately the leakage detector responds to those levels. This process is crucial for ensuring that the leakage detector is properly calibrated and can accurately measure leakage levels within a system.

When checking the calibration of a leakage detector, it's essential to have a tool that can provide varying attenuation across a range of frequencies and levels. The variable attenuator fits this requirement perfectly, as it modifies the power of the signal without introducing significant distortion.

Other tools listed, such as a reflective antenna, digital oscilloscope, and frequency synthesizer, serve different purposes in signal measurement and analysis. While they can be used in conjunction with a leakage detector, they do not allow for the precise calibration and verification of the detector's response to different signal levels in the same way that a variable attenuator can.