Which of the following is not an irrational number 5sqrt5, sqrt2, 4sqrt4, 4sqrt16?

Both $\sqrt{4}$ and sqrt(16 can be written as rational values
$\sqrt{4} = 2$
$\sqrt{16} = 4$
So both $4 \sqrt{4}$ and $4 \sqrt{16}$ are rational.
$5 \sqrt{5}$ and $\sqrt{2}$ are both irrational.