Which one of the following sulphides is yellow?

The highest oxidation state of $Mn$ is exhibited in $Mn_2O_7$. The correct statements about $Mn_2O_7$ are:
$(A)$ $Mn$ is tetrahedrally surrounded by oxygen atoms.
$(B)$ $Mn$ is octahedrally surrounded by oxygen atoms.
$(C)$ Contains $Mn-O-Mn$ bridge.
$(D)$ Contains $Mn-Mn$ bond.
Choose the correct answer from the options given below:

What is the color of $K_2MnO_4$?

What is the chemical formula of Calomel?

Which metal gives a blue borax bead?

Alkaline oxidative fusion of $MnO_2$ gives $A$ which on electrolytic oxidation in alkaline solution produces $B$. $A$ and $B$ respectively are: