Achemical reaction in which two or more substances undergo a chemical union to form a more complex substance is a reaction.
a. double replacement
b. substitution
c. decomposition
d. combination

Answer : The correct option is, (D) Combination

Explanation :

Double replacement reaction : A chemical reaction in which a positive cation and a negative anion of the two reactants exchange places to give two new products or compounds.

        The general representation of this reaction is,

         

         (A and C are the cations, B and D are the anions)

Substitution reaction : A chemical reaction in which a more reactive element displaces the least reactive element.

        The general representation of this reaction is,

         

Decomposition reaction : A chemical reaction in which a bigger compound breaks into two or more smaller compounds or elements.

        The general representation of this reaction is,

         

Combination reaction : A chemical reaction in which two or more substances react to form a more complex substance.

        The general representation of this reaction is,

         

Hence, the correct option is, (D) Combination

Yes you are correct use the scientific method