Which of the following is an example of an irrational number?

An irrational number is defined as a number that cannot be expressed as a simple fraction, meaning it cannot be represented as a ratio of two integers. This type of number has a decimal expansion that neither terminates nor repeats.

The square root of 2, which is approximately 1.41421356..., fits this definition perfectly. It cannot be represented as a fraction of two whole numbers, making it an irrational number. When calculated, the square root of 2 continues infinitely without forming a repeating pattern, solidifying its status as irrational.

The other options provided are examples of rational numbers. For instance, 0.25 can be expressed as the fraction 1/4; the number 3 is an integer that can be represented as 3/1; and 1/3 is itself a fraction. Since these numbers can all be written as a ratio of integers, they do not meet the criteria to be classified as irrational.