The correct answer is D. 2.4 GHz ISM Band.
The 2.4 GHz ISM band is a part of the radio frequency spectrum that is unlicensed for use by the general public. This means that anyone can use it without having to obtain a license from the government. The 2.4 GHz ISM band is used for a variety of applications, including Bluetooth, Wi-Fi, and microwave ovens.
The 2.4 GHz ISM band is a good choice for Bluetooth because it is a relatively wide band with a lot of available bandwidth. This means that Bluetooth devices can communicate with each other quickly and reliably. Additionally, the 2.4 GHz ISM band is a global band, which means that Bluetooth devices can be used in most countries around the world.
The other options are incorrect because they are not unlicensed bands. The Ka Band, L Band, and Ku Band are all licensed bands that are used for a variety of applications, including satellite communications, radar, and weather forecasting.