In alum: ${{\text{K}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}{\text{.Al}}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_{\text{3}}}{\text{.24}}{{\text{H}}_{\text{2}}}{\text{O}}$ which metal can replace ${\text{Al}}$ :
(A) ${\text{Cr}}$
(B) ${\text{Mn}}$
(C) ${\text{In}}$
(D) ${\text{Sc}}$

Hint: The term ‘alum’ is used to refer to a kind of chemical compound and is usually a hydrated double sulfate salt of aluminium. It has the chemical formula ${\text{X}}{\text{.Al}}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_2}{\text{.12}}{{\text{H}}_{\text{2}}}{\text{O}}$ , where X is a monovalent cation like potassium or aluminium. One of the most important alums is potash alum which is simply called alum. It has the chemical formula ${\text{K}}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_2}{\text{.Al}}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_{\text{3}}}{\text{.24}}{{\text{H}}_{\text{2}}}{\text{O}}$ .

Complete step by step answer:
The name alum is used to represent all the hydrated double salts with the same formula and structure. The only difference is that aluminium is replaced by another trivalent metal cation like chromium (III). Sulfur may also be replaced by another chalcogen, i.e., by another group 16 element like selenium.
The most common alum which is an analog of these is the chrome alum which has the chemical formula of ${\text{KCr}}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_2}{\text{.12}}{{\text{H}}_{\text{2}}}{\text{O}}$ . Thus, the metal which can replace aluminium in alum is chromium.

Hence, the correct answer is given by option A.

Additional information:
Potash alum is also called potassium aluminium sulfate. It is a double sulfate of potassium and aluminium. It most commonly occurs in the form of the dodecahydrate ${\text{KAl}}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_2}{\text{.12}}{{\text{H}}_{\text{2}}}{\text{O}}$ .
Potash alum is found to crystallize in regular octahedra with flattened corners. It is very soluble in water.
The potash alum solution is found to be slightly acidic. If the potash alum is neutralized with potassium hydroxide, then alumina or aluminium hydroxide will start to separate.
When heated in red heat, a porous mass known as ‘burnt alum’ is formed.

Note: Potash alum has many applications in medicine and cosmetics.
- It is mainly used as antiseptic in medicine.
- It is used to remove pimples and acne and also in dentistry because of its astringent properties.
- It is also an active ingredient in deodorants as it can inhibit the growth of bacteria responsible for body odor.