Which of the following process is used in the extractive metallugy of magnesium?

The correct option is A Fused salt electrolysis
Calcination:
$MgC{l}_2.6H_{2}O\underset{DryHCl\left(g\right)}{\overset{\Delta \left(calcination\right)}{\to }}MgC{l}_2+5H_{2}O$

It is not made anhydrous by simple heating because it gets hydrolysed.
Extraction of Mg:
$MgC{l}_2.6H_{2}O\stackrel{\Delta }{\to }Mg{O}_5{H}_{2}O+2HCl$

Electrolytic reduction:
Electrolytic reduction of molten anhydrous Carnalite.
$MgC{l}_2\rightleftharpoons M{g}^{2+}+2C{l}^{-}$

At cathode: $M{g}^{2+}+2e^{-}\to Mg$ (99% pure);
At anode: $2C{l}^{-}\to C{l}_2+2e^{-}$

From salt of magnesium, magnesium can not be extracted by other chemical methods as magnesium itself acts as a reducing agent and Mg obtained at cathode during electrolysis of its salt solution, reacts with water forming.

Option A is correct.