The de - Brogile wave length of a neutron in thermal equilibrium with heavy water at a temperature T and mass m , is

Question

$\begin{array}{1 1} (1) \large\frac{h}{\sqrt{mkT}} \\ (2) \large\frac{h}{\sqrt{3 mkT}} \\ (3) \large\frac{2h}{\sqrt{3mkT}} \\ (4) \large\frac{2h}{\sqrt{mkT}} \end{array} $