Which of the following is an irrational number?

An irrational number is a number that cannot be written in the form $p/q$, where $p$ and $q$ are integers and $q \neq 0$. Its decimal expansion is non-terminating and non-recurring. Here, $\sqrt{4} = 2$ (rational), $3.1416$ is terminating (rational), and $1.232323... = 1.\overline{23}$ is non-terminating recurring (rational). Therefore, $\sqrt{5}$ is irrational.