Which of the following is a single replacement reaction? fe2o3 + 3h2o yields 2fe(oh)3
fes + 2hcl yields fecl2 + h2s
2mg + o2 yields 2mgo
mg + 2hcl yields mgcl2 + h2

Explanation:

1. Synthesis reaction is a chemical reaction in which two reactants are combining to form one product.

Example:  

2. Double displacement reaction is one in which exchange of ions take place.  

Example:

3. Single replacement reaction is a chemical reaction in which more reactive element displaces the less reactive element from its salt solution.

Example: