What 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 written in the form of a ratio of two integers. It has a non-repeating and non-terminating decimal expansion.

The square root of 2, which is the correct answer, is indeed an example of an irrational number. Its decimal representation starts as 1.41421356... and continues infinitely without repeating. This characteristic confirms its classification as an irrational number.

In contrast, the other options represent rational numbers. For example, 1/2 is a fraction that can be expressed as 0.5, which terminates. The number 3 is a whole number and can also be represented as the fraction 3/1. Lastly, 0 is also a rational number as it can be expressed as 0/1 or any other fraction where the numerator is zero. Thus, the unique properties of the square root of 2 solidify it as an example of an irrational number.