Which of the following statements is true regarding the given reaction?

3Fe(s)  +  4H₂O(g) →  Fe₃O₄(s) +  4H₂(g)

In the given reaction, iron is oxidized and water acts as an oxidizing agent. 
Fe is gaining oxygen to form Fe₃O₄. The substance that gives oxygen to other compounds, itself gets reduced and acts as an oxidizing agent.

The substance that gains oxygen is said to be oxidized and acts as a reducing agent.

In this reaction, water acts as an oxidizing agent as it is given its oxygen to iron by oxidizing it to produce its oxide. Hence, iron metal is oxidized.