Find the example of the combination reaction?
(a) \[A+B\to AB\]
(b) \[A+{{O}_{2}}\to A{{O}_{2}}\]
(c) \[AB+CD\to AC+BD\]
(d) \[{{O}_{2}}+H\to {{H}_{2}}O\]
(e) \[AB\to A+B\]

Hint: The number of reactants in the combination is not specific, but the number of products in the reaction is specific. Ionic products can also be produced through this reaction.

Complete step by step answer:
Let us understand the combination reaction.
A combination reaction is a type of reaction in which two or more reactants react with each other to form only a single product. This reaction is also known as a synthesis reaction.
The general representation of the combination reaction is:
\[X+Y\to XY\]
There can be any number of reactants in the reaction.
The nature of the combination reaction is usually exothermic. This is the due fact that when the reactants combine to form products the energy is released due to the formation of bonds.
For example, the burning of coal
\[C+{{O}_{2}}\to C{{O}_{2}}\]
When one reactant is metal and the other reactant is non-metal then the product formed is an ionic product.
 Now let us observe all the options one by one:
(a) \[A+B\to AB\]
This reaction has only one product. Hence, it is a combination reaction.
(b) \[A+{{O}_{2}}\to A{{O}_{2}}\]
This reaction also has one product. Hence, it is also a combination reaction.
(c) \[AB+CD\to AC+BD\]
This reaction has two products. It is not a combination reaction. It is an example of a displacement reaction.
(d) \[{{O}_{2}}+H\to {{H}_{2}}O\]
This reaction also has one product. Hence, it is also a combination reaction.
 (e) \[AB\to A+B\]
This reaction has two products. It is not a combination reaction.
Hence, the correct answers are options (a), (b), and (d).

Note: You may get confused that option (e) is also a combination reaction because it has one reactant. Only the product has to be one. Even if in the reaction product + heat, then also it is a combination reaction.