Explanation: The VOR (VHF Omnidirectional Range) navigation system determines an aircraft's bearing by comparing the phase of two 30 Hz signals. One is a reference signal, omnidirectional and constant in phase. The other is a variable signal, created by a rotating directional antenna pattern, which completes 30 full rotations per second. As this variable signal rotates, its phase changes relative to the reference signal. An aircraft's receiver measures this phase difference, which directly translates into its magnetic bearing from the VOR station. Therefore, the transmitted VOR variable signal rotates at a rate of 30 revolutions per second. Options A, C, and D are incorrect as they do not correspond to the internationally standardized rotation rate for VOR variable signals.