Identify the redox reactions among the following and determine the oxidizing and reducing agents in them:
$(a) 3HCl_{(aq)} + HNO_{3_{(aq)}} \to Cl_{2_{(g)}} + NOCl_{(g)} + 2H_2O_{(l)}$
$(b) HgCl_{2_{(aq)}} + 2KI_{(aq)} \to HgI_{2_{(s)}} + 2KCl_{(aq)}$
$(c) Fe_2O_{3_{(s)}} + 3CO_{(g)} \xrightarrow{\Delta} 2Fe_{(s)} + 3CO_{2_{(g)}}$

(A, C) To identify a redox reaction,we check for changes in the oxidation numbers of the elements involved.
$(a) 3HCl + HNO_3 \to Cl_2 + NOCl + 2H_2O$
In this reaction,the oxidation number of $Cl$ increases from $-1$ to $0$ (oxidation),and the oxidation number of $N$ decreases from $+5$ to $+3$ (reduction). Thus,it is a redox reaction. $HCl$ is the reducing agent and $HNO_3$ is the oxidizing agent.
$(b) HgCl_2 + 2KI \to HgI_2 + 2KCl$
In this reaction,the oxidation numbers of all elements $(Hg: +2, Cl: -1, K: +1, I: -1)$ remain unchanged. Thus,it is not a redox reaction.
$(c) Fe_2O_3 + 3CO \to 2Fe + 3CO_2$
In this reaction,the oxidation number of $Fe$ decreases from $+3$ to $0$ (reduction),and the oxidation number of $C$ increases from $+2$ to $+4$ (oxidation). Thus,it is a redox reaction. $CO$ is the reducing agent and $Fe_2O_3$ is the oxidizing agent.