The correct answer is A. 2 kHz.
Radio waves are a type of electromagnetic radiation with wavelengths longer than visible light. They are used for communication, navigation, and astronomy. The frequency of a radio wave is the number of waves that pass a given point in space per second. The higher the frequency, the shorter the wavelength.
Radio waves are classified into bands based on their frequency. The lowest frequency band is the longwave band, which has frequencies from 30 to 300 kHz. The highest frequency band is the microwave band, which has frequencies from 1 GHz to 300 GHz.
The frequency of 2 kHz is the lowest frequency that is considered to be a radio wave. This is because radio waves with frequencies below 2 kHz are not able to propagate through the atmosphere. They are absorbed by the Earth’s surface and cannot be used for communication.
The other options are incorrect because they are frequencies that are not considered to be radio waves. Option B, 10 kHz, is the lowest frequency that is used for commercial AM radio. Option C, 50 kHz, is the lowest frequency that is used for commercial FM radio. Option D, 200 kHz, is the lowest frequency that is used for marine radio.