What must occur for a signal to travel through air?

For a signal to travel through air, it is essential for it to be converted from electrical energy to electromagnetic energy. This process is fundamental because air is not conducive to the transmission of electrical signals in their raw form; instead, electromagnetic waves, such as radio waves, are capable of propagating through the air. The conversion to electromagnetic energy allows the signal to move through the vast medium of air, where it can be received by antennas or other devices designed to detect these waves.

In contrast, amplification of a signal is not a prerequisite for its travel through air; signals can naturally dissipate without needing to be enhanced. While cables are effective in transmitting electrical signals, they are not necessary when transmitting signals through air, as the required transmission medium is the air itself. Encryption, while important for securing data, does not directly impact the ability of a signal to travel through air. The key aspect here is the transformation of the signal into a form that can traverse the air medium, making the conversion to electromagnetic energy the correct answer.