A combination (or synthesis) reaction is a type of chemical reaction in which two or more reactants (elements and/or compounds) combine to form a single new product. It is represented generally as:

A + B → AB

This is one of the fundamental classifications of chemical reactions (alongside decomposition, displacement, and double displacement reactions), and it is defined purely by the pattern of reactants combining into one product — it says nothing about whether heat is absorbed or released.

The statement that is incorrect is: "All combination reactions are endothermic in nature." This is false because combination reactions can be either exothermic (releasing energy, usually as heat) or endothermic (absorbing energy), and in practice the vast majority of common combination reactions encountered in chemistry are exothermic. Examples include:

• CaO (quicklime) + H₂O → Ca(OH)₂ (slaked lime) + heat — a strongly exothermic reaction used practically to generate heat.
• 2H₂ + O₂ → 2H₂O + heat — combustion of hydrogen releases a large amount of energy.
• N₂ + 3H₂ → 2NH₃ + heat (the industrial Haber process for ammonia synthesis), which is exothermic, though it requires a catalyst and specific pressure/temperature conditions to proceed at a useful rate.
• C + O₂ → CO₂ + heat, as occurs when coal burns.

Only a limited number of combination reactions are endothermic (for instance, the combination of nitrogen and oxygen to form nitric oxide, N₂ + O₂ → 2NO, requires a continuous supply of energy such as lightning or a spark). This confirms that heat change is not a defining or universal feature of combination reactions — some release heat, some absorb it.

Why the other statements are correct (and hence not the answer):

• The burning of coal (C + O₂ → CO₂) is indeed a valid, textbook example of a combination reaction, since carbon and oxygen combine to give a single product.
• By definition, a combination reaction does produce a single product from two or more reactants, which correctly describes the general pattern A + B → AB.
• CaO reacting with water to form calcium hydroxide with release of heat is factually correct and is a classic exothermic combination reaction used in lime-slaking.

Since the question asks for the incorrect statement, the claim that all combination reactions are endothermic is the one that does not hold true.