What does the use of two frequencies in secondary radar transactions eliminate?

Using two frequencies in secondary radar transactions is primarily aimed at improving the accuracy and reliability of the radar system. One of the key benefits of this approach is the elimination of clutter caused by direct reflections. This is crucial because direct reflections can interfere with the radar signals and make it difficult to distinguish the aircraft's transponder signals from other unwanted signals in the environment.

By employing two different frequencies, the secondary radar system can differentiate between the desired return signals from the transponders and the unwanted reflections from other surfaces, such as terrain or weather phenomena. This frequency diversity helps to reduce the impact of false echoes and clutter, enabling more precise tracking of aircraft.

The other options, while they may be relevant to radar operations, do not directly pertain to the primary effect that the use of two frequencies achieves in this context.