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

Irrational numbers are numbers that cannot be expressed as a fraction of two integers, meaning their decimal expansions are non-repeating and non-terminating. The value of π (pi) is a well-known example of an irrational number; it is approximately equal to 3.14159 and continues infinitely without repeating.

In contrast, the other choices represent rational numbers. For example, 1/2 and 0.5 can be expressed as fractions of integers (1/2 is already a fraction, and 0.5 equals 1/2 when converted), while -1 can also be expressed as -1/1, qualifying it as a rational number as well. Thus, the correct answer highlights a number that distinctly does not fit the category of rational numbers, which is why π is the exemplary choice for an irrational number.