Oxidizing agent: An oxidizing agent is an element that gains electrons. Since, the oxidizing agent means to gain electrons; it is said to have been reduced.

The element which undergoes reduction (gets reduced) is called oxidizing agent. 

(a) Pb₃O₄ is reducing agent 

Pb₃O₄ + 8HCl → 3PbCl₂ + Cl₂ + 4H₂O

In the given reaction, Pb₃O₄ loses oxygen and reduces to PbCl₂. Thus, Pb₃O₄ is an oxidizing agent as it undergoes reduction. 

(b) O₂ is oxidizing agent 

2Mg + O₂→ 2MgO 

In the given reaction, O₂ is reduced by losing oxygen atoms. Thus, O₂ is an oxidizing agent as it undergoes reduction. 

(c) CuSO₄ is oxidizing agent 

CuSO₄ + Zn → Cu + ZnSO₄ 

In the given reaction, CuSO₄ is reduced to Cu by losing oxygen atoms. Thus, CuSO₄ is an oxidizing agent as it undergoes reduction.

(d) V₂O₅ is oxidizing agent 

V₂O₅ + 5Ca → 2V + 5CaO 

In the given reaction, V₂O₅ is reduced to V by losing oxygen atoms. Thus, V₂O₅ is an oxidizing agent as it undergoes reduction. 

(e) H₂O is oxidizing agent 

3Fe + 4H₂O → Fe₃O₄ + 4H₂

In the given reaction, H₂O is reduced to H₂ by losing oxygen atoms. Thus, H₂O is an oxidizing agent as it undergoes reduction. 

(f) CuO is oxidizing agent 

CuO + H₂→ Cu + H₂O 

In the given reaction, Cu is reduced to Cu by losing oxygen atom. Thus, CuO is an oxidizing agent as it undergoes reduction.