Which of the following has conductivity and appearance like that of copper metal?

Which from the following is the general formula of the compound obtained when lanthanoid $(Ln)$ reacts with carbon at elevated temperature?

Highest fluoride and highest oxide of $Mn$ are respectively

Iron on reacting with carbon gives:

$A \xrightarrow{4 KOH, O_2} 2 B (Green) + 2 H_2O$
$3 B \xrightarrow{4 HCl} 2 C (Purple) + MnO_2 + 2 H_2O$
$3 C \xrightarrow{H_2O, KI} 2 A + 2 KOH + D$
In the above sequence of reactions,$A$ and $D$,respectively,are:

How is sodium chromate converted into sodium dichromate in the manufacture of potassium dichromate from chromite ore?