What characteristic defines the binary number system?

The binary number system is characterized by its representation of data using only two symbols: 0 and 1. This foundational aspect allows computers and digital systems to function, as they interpret these binary digits (bits) as electrical states: on (1) and off (0). The simplicity of having just two symbols supports efficient data processing and storage in binary formats, making it essential to computing and digital communications.

The other options present different numbering systems or representations. For example, the option mentioning base 10 refers to the decimal system, which is not applicable to binary. The discussion of combining letters and symbols relates more to other encoding systems, like ASCII, and the last option regarding having 16 different values is indicative of the hexadecimal system. Thus, the defining characteristic of the binary number system is its unique use of only 0s and 1s.