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

An irrational number is one that cannot be expressed as a simple fraction or as the ratio of two integers. Instead, its decimal representation goes on forever without repeating. Among the options, π (pi) fits this definition perfectly. It is a mathematical constant that represents the ratio of a circle's circumference to its diameter and is known to be approximately 3.14159. However, its decimal expansion continues infinitely without settling into a repeating pattern, making it an irrational number.

In contrast, the other choices can be represented as either terminating decimals or repeating decimals. For instance, 1/2 is a terminating decimal (0.5), 0.333... is a repeating decimal that equals 1/3, and 5 is a whole number. All of these options can be expressed as ratios of integers, confirming their status as rational numbers. Therefore, π is the only one among the options that is classified as an irrational number.