The correct IUPAC name of K 2 MnO 4 is:

The structure of the IUPAC name is 

name of cationic part + Name of anionic part.

Metal present in the anionic complex, so 'ate', must be added at the end of the metal name. 

Oxidation state of $\mathrm{Mn}$

$$\begin{gathered} \mathrm{Mn}+4\left(-2\right)=-2 \\ =+6 \end{gathered}$$

The structure of anionic complex name is

ligand names + metal name(oxidation state)

${\mathrm{K}}_2{\mathrm{MnO}}_4$

$2+\mathrm{x}–8=0$

$\Rightarrow \mathrm{x}=+6$

Oxidation state of $\mathrm{Mn}=+6$

IUPAC Name is:

Potassium tetraoxidomanganate $\left(\mathrm{VI}\right)$