Answer: D — Irrational number
$\sqrt{0.4} = \sqrt{\dfrac{2}{5}} = \dfrac{\sqrt{2}}{\sqrt{5}}$, which cannot be expressed as $\dfrac{p}{q}$ (integers), so it is an irrational number.
$0.4 = \frac{2}{5}$, and since 2 and 5 are primes, $\sqrt{2}$ and $\sqrt{5}$ are both irrational. The quotient of two irrationals here does not simplify to a rational — the result is still irrational. The key rule from the chapter: $\sqrt{p}$ is irrational when $p$ is prime, and products/quotients involving irrationals (with non-zero rationals) remain irrational. Examiners expect you to simplify $\sqrt{0.4}$ first, then identify its nature.