What number system consists of only ones and zeros?

The binary number system is the correct choice because it is a base-2 numeral system that exclusively uses two symbols: 0 and 1. This simplicity allows for straightforward representation of data in digital computing, where each binary digit, or bit, corresponds to an on/off state. The binary system underlies nearly all modern computing and digital communications, making it fundamental in areas such as programming, data storage, and network transmission.

In contrast, the decimal number system, which is the most commonly used by humans, is a base-10 system that incorporates ten digits (0-9). The hexadecimal system is a base-16 system that utilizes sixteen symbols (0-9 and A-F) for a broader range of values, while the octal number system is base-8, utilizing eight digits (0-7). Each of these systems serves particular purposes within computing and mathematics but does not exclusively use just ones and zeros like the binary system does.