In which of these substances did the individual atoms combine together in a chemical reaction?. Single choice.

hello the options related to your question is missing hence I will give you a general answer as regards to combination of atoms in a chemical reactions

answer :

In a chemical reaction the bonds that are found between the atoms of reactants are broken up and a different/new bonds are formed between the atoms of the various reactants involved in the chemical reaction to produce a new product.   during this process no new atoms are created but new bonds are created  to form a product

Example of such substance where atoms can combine together in a chemical reaction is : Iron sulfide  [ Fe(s) + S(s) → FeS(s) ]

Explanation:

In a chemical reaction the bonds that are found between the atoms of reactants are broken up and a different/new bonds are formed between the atoms of the various reactants involved in the chemical reaction to produce a new product.   during this process no new atoms are created but new bonds are created  

Example of such substance where atoms can combine together in a chemical reaction is : Iron sulfide  [ Fe(s) + S(s) → FeS(s) ]  i.e. combination of Iron atoms and sulfur atoms