<u>Answer:</u> The correct answer is Synthesis and Combustion reaction.
<u>Explanation:</u>
Combustion reaction is defined as the chemical reaction in which a hydrocarbon reacts with oxygen to produce carbon dioxide and water.

Decomposition reaction is defined as the chemical reaction in which a single substance breaks down into two or more simpler substances.

Double displacement reaction is defined as the chemical reaction in which exchange of ions takes place.

Single displacement reaction is defined as the chemical reaction in which more reactive element displaces a less reactive element from its chemical reaction.

Synthesis reaction is defined as the chemical reaction in which substances combine in their elemental state to form a single compound.

- <u>For A:</u>

This reaction is considered as a synthesis reaction because hydrogen gas and chlorine gas are combining in their elemental states to produce hydrochloric acid molecule.
- <u>For B:</u>

This reaction is considered as a combustion reaction because methane gas is reacting with oxygen gas to produce carbon dioxide gas and water molecule.
Thus, the correct answer is Synthesis and combustion reaction.