Which letter is not used in the hexadecimal number system?

The hexadecimal number system is a base-16 system that uses the digits 0 through 9 and the letters A through F to represent values. In this system, the letters A, B, C, D, E, and F represent the decimal values 10, 11, 12, 13, 14, and 15 respectively. Therefore, the only letter listed in the choices that is not used in the hexadecimal system is G. This makes G the correct choice, as it does not have a corresponding value in hexadecimal representation. The other letters—A, C, and F—are valid components of the hexadecimal system and represent numbers within its range.