Question

In which type of chemical reaction do two or more substances combine to form one substance?

Answer 1

the answer is 
synthesis 

Answer 2

Explanation:

A reaction in which two or more substances combine together to form a single product is known as synthesis reaction.

For example, $C + O_{2} \rightarrow CO_{2}$

     or,        $H_{2} + \frac{1}{2}O_{2} \rightarrow H_{2}O$

During synthesis reaction, heat or light are released in the form of energy. Therefore, synthesis reactions are exothermic in nature.