Which one of the following is not an ore of aluminium?
(a) Bauxite
(b) Corundum
(c) Epsomite
(d) Cryolite

Hint: An ore of aluminium is the one which contains higher amounts of aluminum along with other compounds such as sulphates, hydrates, etc.

Complete step by step answer:
Let us try to understand the word ‘ore’.
“Ore is natural rock or sediment that contains one or more valuable minerals, typically metals, that can be mined, treated and sold at a profit. Ore is extracted from the earth through mining and treated or refined, often via smelting, to extract the valuable metals or minerals”. So, an ore of an aluminium is one which contains aluminium (mostly) along with some other elements.
Looking at the options, we have –
Bauxite - \[A{{l}_{2}}{{O}_{3}}.2{{H}_{2}}O\]
It is the main source of aluminium. It is composed of mainly aluminium bearing minerals.
Bauxite forms when laterite soils are severely leached of silica and other soluble materials in a wet tropical or subtropical climate.
It occurs as a reddish-brownish rock.
Corundum - \[A{{l}_{2}}{{O}_{3}}\]
It is an aluminium oxide. It occurs as a crystal in many colors. Along with aluminium, it contains traces of iron, titanium, vanadium and chromium.
Epsomite - \[MgS{{O}_{4}}.7{{H}_{2}}O\]
It is a hydrous magnesium sulfate mineral. It occurs as a colorless to white crystal with tints of yellow, green and pink.
Cryolite - \[N{{a}_{3}}Al{{F}_{6}}\]
It is a very rare aluminium halide mineral, which occurs as a colourless to white crystal.
Therefore, the answer is – option (c) – Epsomite is not an ore of aluminium.

Note: Other Aluminium ores include –
Felspar - \[KAlS{{i}_{3}}{{O}_{8}}\]
It is an aluminosilicate mineral.
Alunite - \[{{K}_{2}}S{{O}_{4}}.A{{l}_{2}}{{(S{{O}_{4}})}_{3}}.4Al{{(OH)}_{3}}\]
It is a hydroxylated aluminium potassium sulfate mineral
Kaolin - \[3A{{l}_{2}}{{O}_{3}}.6Si{{O}_{2}}.2{{H}_{2}}O\]
It is a silicate form of aluminium. It occurs as a clay.