Explanation:

The binary number system is a base-$2$ system, meaning it employs exactly two distinct digits to represent all numbers. These digits are universally $0$ and $1$. For instance, the decimal number $2$ is $10_2$ in binary, using two digits. Option (b) Two is correct. Option (a) One is incorrect; a system with one digit could only represent zero. Options (c) Three and (d) Four are incorrect, as they would correspond to ternary (base-$3$) or quaternary (base-$4$) systems, respectively. The fundamental characteristic of "binary" is "two" distinct symbols.