Which of the following statements about base numbers is true?

Base numbers are fundamental in numeral systems and are defined as the number of unique digits used to represent numbers in that system. For instance, in the decimal system (base 10), there are ten unique digits (0-9). In binary (base 2), there are two unique digits (0 and 1). Therefore, stating that base numbers define the number of unique digits is accurate and reflects a core aspect of how various numeral systems work.

The other options present misconceptions about the role and properties of base numbers. For example, base numbers do not need to be prime; they can be any positive integer greater than 1. Additionally, there are many numeral systems with bases less than 10, such as binary and octal, which proves that base numbers do not have to be greater than 10. Lastly, base numbers are unrelated to the order of operations in mathematics, which is governed by standard mathematical rules like PEMDAS (Parentheses, Exponents, Multiplication and Division, Addition and Subtraction). Thus, the correct statement highlights the essential characteristic of base numbers in numeral systems.