Explanation:

The question asks about the number system used *in* binary numbers. By definition, the binary number system is a base-$2$ system that exclusively uses two symbols, typically $0$ and $1$, to represent all numerical values. Therefore, the binary number system itself is used. Option (b) Binary is correct. Option (a) decimal refers to the base-$10$ system. Option (c) Byte is a unit of digital information (8 bits). Option (d) Bit is the smallest unit of digital information (a single $0$ or $1$). These are related concepts but do not describe the number system itself.