a) Roasting and calcination are the two processes to reduce the metal oxides to the respective metals.
(b) Magnesium is extracted from its ore by electrolytic reduction process.
(c) Cryolite is used in metallurgy of aluminium in order to increase the fusibility and conductivity
(d) I Hall's process bauxite is leached with the help of \[N{a_2}C{O_3}\] ​.
Which of the following options regarding the above statements is correct?
A. a, b and c
B. b, c and d
C. c and d
D. only a

Hint: Calcination is the method to form oxides of metals, Roasting is the method to form oxide of sulphurs, electrolytic reduction is separation due to electrical charge and I-hall process is the alumina separation.

Complete step by step answer:
In order to pick odd one out now we will study about each process mentioned,
Calcination is the process by which ore is converted into oxides by strong heating. In the procedure of calcination volatile impurities and moisture from ore are removed by heating them in low supply or absence of oxygen, and the ore is heated below its melting point either in absence of air or in limited supply. This method is commonly used for producing oxides from carbonates and hydroxides.
Example: Conversion reaction of calcium carbonate into carbon-dioxide.
\[CaC{o_3}\; \to {\text{ }}CaO{\text{ }} + {\text{ }}C{O_2}\]
Roasting: it is the process of converting sulphide ores into its oxide form by heating the ore in the presence of oxygen, above its melting point. The process of roasting involves certain reactions such as reduction, sulfation, oxidation, pyro hydrolysis and chlorination.
Example: conversion reaction of zinc sulphide into its oxide.
\[2ZnS + 3{O_2}\; \to {\text{ }}2ZnO{\text{ }} + {\text{ }}C{O_2}\]
Electrolytic reduction: It is the process of electrolytic separation of metals from its ore, in which metals get collected at cathode and other ions get collected at anode.
Example: Separation of magnesium from its ore.
Cryolite: It is a white powdered mineral used in the metallurgy of aluminium as its addition reduces its melting temperature and increases its conductivity.
I-Hall's process: It is the process by which impurities are removed from bauxite ore to extract alumina with the leaching reagent sodium carbonate.

So, the correct answer is “Option B”.

Note:
During the process of roasting sulphides are converted to their oxides which are a source of air pollution, and the major setback of this process is that they release acidic and toxic compounds which cause harm to the atmosphere.