Which of the following are Irrational Numbers?

I too would limit it to real numbers, but I find three of them:

π/4, 4/π, √(21)

Irrational numbers are real numbers that cannot be represented as the ratio of two integers.

So if you take out the complex numbers and the rational numbers, what you'll have left are the irrational ones.

Watch out for expressions that can simplify into rational expressions.  √4, while it has a radical on it, is rational as it can simplify to 2.

The same with 0/π.

π/0 isn't defined so it can't be classified in any number system.

So once you take out the rational, complex, and undefined numbers, what is left with are irrational:

π/4, √21

Those are the only two numbers that are irrational.