Which of the following real numbers is NOT irrational (i.e., is rational)?

Step 1 (Define rational and irrational):

• A real number is rational if it can be written in the form $\frac{p}{q}$ where $p$ and $q$ are integers and $q\ne 0$.

• A real number is irrational if it cannot be written in that form.

Step 2 (Analyze the square roots):

• $\sqrt{2},\sqrt{3},\sqrt{5}$ are square roots of prime numbers (which are not perfect squares). They are well-known to be irrational numbers.

• $\sqrt{4}$ is the square root of a perfect square ($2^2=4$):

Since $2=\frac{2}{1}$, it is a rational number, meaning it is NOT irrational.

Step 3 (Conclusion): The number $\sqrt{4}$ is rational, so it is not irrational. This corresponds to option index 0.