Final answer:
Reaction 4, the reaction of iron with oxygen to form iron(II) oxide (2Fe + O₂ → 2FeO), is both a combustion and a synthesis reaction. The correct answer is option d.
Step-by-step explanation:
The student has asked which of the four given chemical reactions is an example of a combustion and also a synthesis reaction. A combustion reaction involves a substance reacting with oxygen, often producing heat and light, and typically results in the formation of water and carbon dioxide when an organic compound is burnt.
A synthesis reaction, on the other hand, involves the combination of two or more substances to form a new single compound. The correct answer is d. Reaction 4, which is the reaction 2Fe + O₂ → 2FeO. This is a synthesis reaction because iron (Fe) and oxygen (O₂) combine to form a single compound, iron(II) oxide (FeO).
It is also a combustion reaction as iron reacts with oxygen, which is a form of combustion. The other reactions listed do not represent both combustion and synthesis simultaneously.