Identify the last step in wet chemical synthesis of nanomaterial.

Match List-$I$ with List-$II$ and choose the correct answer from the options given below:

List-$I$ (Name of the test)	List-$II$ (Reaction sequence involved) [$M$ is metal]
$A$. Borax bead test	$I$. $MCO_3$ $\rightarrow MO$ $\xrightarrow[+\Delta]{Co(NO_3)_2} CoO \cdot MO$
$B$. Charcoal cavity test	$II$. $MCO_3$ $\rightarrow MCl_2$ $\rightarrow M^{2+}$
$C$. Cobalt nitrate test	$III$. $MSO_4$ $\xrightarrow[\Delta]{Na_2B_4O_7} M(BO_2)_2$ $\rightarrow MBO_2$ $\rightarrow M$
$D$. Flame test	$IV$. $MSO_4$ $\xrightarrow[\Delta]{Na_2CO_3} MCO_3$ $\rightarrow MO$ $\rightarrow M$

What is the formula of microcosmic salt?

Which of the following gives the chromyl chloride test?

Which pair of cations can be identified by the borax bead test?

Given below are two statements:
Statement $I$: Colourless cupric metaborate is reduced to cuprous metaborate in a luminous flame.
Statement $II$: Cuprous metaborate is obtained by heating boric anhydride and copper sulphate in a non-luminous flame.
In the light of the above statements,choose the most appropriate answer from the options given below.