Magnesium does not decompose in the
A. Steam
B. Hot water
C. Cold Water
D. Semi-hot water

Hint: Magnesium is highly reactive. It reacts with water and acids quickly. Normally magnesium in reaction with water gives magnesium hydroxide and hydrogen gas. Magnesium reacts with hot water and forms metal hydroxide but not with cold water.

Complete Step by Step Answer:
Magnesium belongs to group 2 and period 3. It is an alkaline earth metal.
It is very reactive.
We have to find out whether or not it decomposes the given options.
A. Steam
Magnesium in contact with steam yields magnesium oxide and hydrogen gas.
The reaction happens as follows:
\[{\rm{Mg}}\left( {\rm{s}} \right){\rm{ + }}{{\rm{H}}_{\rm{2}}}{\rm{O}}\left( {\rm{s}} \right) \to {\rm{MgO}}\left( {\rm{s}} \right){\rm{ + }}{{\rm{H}}_{\rm{2}}}\left( {\rm{g}} \right)\]
So, A is incorrect.
B. Hot water
Magnesium reacts with hot water readily to form magnesium hydroxide and hydrogen gas.
The reaction happens as follows:
\[{\rm{Mg}}\left( {\rm{s}} \right){\rm{ + 2}}{{\rm{H}}_{\rm{2}}}{\rm{O}}\left( {\rm{l}} \right) \to {\rm{Mg}}{\left( {{\rm{OH}}} \right)_{\rm{2}}}\left( {\rm{s}} \right){\rm{ + }}{{\rm{H}}_{\rm{2}}}\left( {\rm{g}} \right)\]
​So, B is incorrect.
C. Cold Water
Magnesium doesn't react with cold water. After a considerable amount of time, some bubbles of hydrogen are present on the surface of the metal.
This reaction ceases as the magnesium hydroxide yielded is practically insoluble in water and yields an obstacle for further reaction.
This is because it begins to float as the bubbles of hydrogen gas stick to its surface.
The reaction happens as follows:
\[{\rm{Mg}}\left( {\rm{s}} \right){\rm{ + 2}}{{\rm{H}}_{\rm{2}}}{\rm{O}}\left( {\rm{l}} \right) \to {\rm{Mg}}{\left( {{\rm{OH}}} \right)_{\rm{2}}}\left( {\rm{s}} \right){\rm{ + }}{{\rm{H}}_{\rm{2}}}\left( {\rm{g}} \right)\]
So, C is correct.
D. Semi-hot water
Magnesium reacts with hot water readily to form magnesium hydroxide and hydrogen gas.
The reaction happens as follows:
\[{\rm{Mg}}\left( {\rm{s}} \right){\rm{ + 2}}{{\rm{H}}_{\rm{2}}}{\rm{O}}\left( {\rm{l}} \right) \to {\rm{Mg}}{\left( {{\rm{OH}}} \right)_{\rm{2}}}\left( {\rm{s}} \right){\rm{ + }}{{\rm{H}}_{\rm{2}}}\left( {\rm{g}} \right)\]
​So, D is incorrect.

So, option D is correct.

Note: Metals on reaction with water form metal oxide and hydrogen gas. Metal oxides that are soluble in water undergo dissolution to form metal hydroxide. But all metals do not exhibit this property. Alkaline earth metals other than Mg in reaction with water to give hydrogen and a metal hydroxide. This reaction is slow. Beryllium is an exception.