Final Answer:
The number classified as a rational number among the options provided is 2. Therefore, the correct option is d. 2.
Step-by-step explanation:
Rational numbers are those that can be expressed as fractions of two integers, where the denominator isn't zero. In this case, the number "2" fits the criteria. It can be written as the fraction 2/1, where both the numerator and denominator are integers. Rational numbers include integers, fractions, terminating decimals, and repeating decimals. While options a, b, and c (17, 23, 9) are integers, they don't necessarily fit the definition of a rational number as they're whole numbers without a fractional part.
The fundamental characteristic of rational numbers is their expressibility as a ratio of two integers. For instance, 17 can be written as 17/1 or 23 as 23/1, but since they lack a fractional part, they are integers rather than rational numbers. Similarly, the number 9, although an integer, doesn't qualify as a rational number since it's not expressed as a fraction of two integers.
Understanding the concept of rational numbers involves recognizing that they encompass various forms, including whole numbers (which are a subset of integers) and fractions, where the denominator and numerator are integers. Therefore, the correct option is d. 2.