The number that is irrational is √19.
An irrational number is a number that cannot be expressed as a ratio of two integers (a fraction) and it cannot be represented as a repeating or terminating decimal. The square root of 19 is an irrational number because it cannot be expressed as a ratio of two integers. It is non-repeating, non-terminating decimal that goes on forever, so it cannot be represented as a decimal.