Which of the following frequency bands is commonly used for unlicensed spectrum?

The 2.4 GHz frequency band is commonly used for unlicensed spectrum because it is designated for open access and is specifically allocated for various wireless communications technologies, including Wi-Fi and Bluetooth. This band is part of the industrial, scientific, and medical (ISM) radio bands, which are set aside by international agreements for unlicensed use. Devices operating in this band do not require individual licenses, making it widely accessible for consumer electronics and enabling various applications without the need for coordination with regulatory bodies.

In contrast, the other frequency bands mentioned have different usages. The 800 MHz and 700 MHz bands are primarily designated for licensed telecommunications services, including cellular and some public safety communications, which require specific licenses for operation due to potential interference issues. The 1.8 GHz band is also used for licensed communications, primarily in mobile networks. Therefore, the distinct characteristics of the 2.4 GHz band enable its widespread adoption for numerous unlicensed applications, making it the correct choice in this context.