The correct answer is A. 2.
The binary system is a numeral system with two as its base. It is the simplest numeral system, and is often used to represent data in computers and other electronic devices. In binary, each digit can be either 0 or 1, and these digits are used to represent numbers in a series of powers of 2. For example, the number 10 in binary is represented as 1010, which is equal to 2^3 + 2^1 + 2^0 = 8 + 2 + 1 = 11 in decimal.
The other options are incorrect because they are not powers of 2. Option B, 10, is the base of the decimal system, which is the most common numeral system used in the world. Option C, 8, is the base of the octal system, which is sometimes used in computer programming. Option D, 16, is the base of the hexadecimal system, which is also sometimes used in computer programming.