The correct answer is C. 2.
A radix, also called base, is the number of unique digits used to represent numbers in a positional numeral system. The most common radix is 10, which is used in the decimal system. Other common radices are 2, which is used in the binary system, and 8, which is used in the octal system.
In a binary number, each digit can be either 0 or 1. The position of each digit determines its value, with the rightmost digit having a value of 1, the next digit to the left having a value of 2, and so on. For example, the binary number 1011 is equal to 123 in decimal, because 1 x 2^3 + 0 x 2^2 + 1 x 2^1 + 1 x 2^0 = 8 + 0 + 2 + 1 = 11.
The radix of a binary number is 2, because there are two unique digits used to represent numbers in the binary system.