Which of the following melts in boiling water?

The fusion of manganese dioxide with $KOH$ in the presence of an oxidizing agent like $KNO_3$ yields a compound. Identify the compound and its color.

Consider the following reactions:
$K_2Cr_2O_7$ $\xrightarrow[-H_2O]{KOH} [A]$ $\xrightarrow[-H_2O]{H_2SO_4} [B] + K_2SO_4$
The products $[A]$ and $[B]$,respectively,are:

$KMnO_4$ decomposes on heating at $513 \ K$ to form $O_2$ along with

$Cu^{2+}$ aqueous solution has . . . . . . colour.

Which of the following is used as a white pigment?