Which one of the following is not an endothermic reaction?

A chemical reaction is termed exothermic if the energy in the form of heat is released in the reaction. An endothermic reaction is a reaction taking place by using heat energy.

In the conversion of graphite to diamond, a high temperature is used and thus, it is an endothermic reaction.
The decomposition reaction of water is the breaking of water molecules. To break the bond energy is required. So, it is an endothermic reaction.
Dehydrogenation of ethane involves the removal of the dihydrogen molecule. Here also bond cleavage is taking place and is an endothermic reaction.
In the combustion of methane, methane is burned to give CO₂ and heat. In simpler words, the combustion of any substance produces heat. And therefore is an exothermic reaction.
Therefore, the reaction which is not endothermic is the combustion of methane.

Hence, the correct answer is option D.