The main ore used for the extraction of iron is:
A.Haematite
B.Calamite
C.Bauxite
D.Cryolite

Hint: To answer this question you must recall the process of extraction of metals from their ores and the ores of various metals. An ore is a mineral containing the required metal in the form of a compound with many impurities along with it. The process of the treatment of this mineral to obtain pure metal is known as metallurgy.

Complete answer:
We first study each of the given ores one by one in order to find the answer.
Option A is Haematite. The name haematite is derived from the Greek word haima which means blood in accordance with the red colour of the ore due to the presence of iron (II) oxide or ferric oxide. Thus, haematite is an ore of iron.
Option B is Calamite. Calamite is an ore of zinc containing two different minerals present in the ore distinctly. The two minerals are smithsonite (zinc carbonate) and hemimorphite (zinc silicate).
Option C is Bauxite. Bauxite is an ore of aluminium and gallium. It consists of various minerals like gibbsite (aluminium hydroxide), diaspore and boehmite, along with impurities of minerals of other metals namely haematite, anatase, kaolinite, goethite and ilmenite.
Option D is Cryolite which is also an ore of aluminium used for the extraction of aluminium in the Hall- Heroult process. Thus, the ore among the following used for the extraction of iron is haematite.

Thus, the correct answer is A.
Note:
In the extraction process of iron, haematite ore is heated in a blast furnace with carbon as coke and limestone. The iron oxide is reduced by carbon and iron is obtained.