What is the general range of usable radio frequencies?

The general range of usable radio frequencies is primarily defined by the electromagnetic spectrum allocated for communication purposes. The correct range is from approximately 3 kHz to 400 GHz. This encompasses very low frequencies used for longwave radio broadcasts to ultra-high frequencies utilized in modern communication technologies such as satellite and microwave communication.

The lower limit, 3 kHz, marks the transition from extremely low frequencies to those typically classified as radio waves, which are essential for practical communication applications. Frequencies below this threshold are generally not used for radio transmission due to their inefficiency for such purposes and regulatory limitations.

The upper limit of 400 GHz includes various types of data transmissions, including those used for radar, satellite signals, and other advanced communication systems. Frequencies above this point typically enter the infrared and optical regions of the spectrum, which are not considered radio frequencies and are utilized for different applications.

The other options listed either start at too low of a frequency, include ranges that aren't commonly utilized for radio communications, or extend beyond the definition of what constitutes usable radio frequencies in practical terms.