Question

In this unbalanced reaction, which atom is reduced? h2o + cl2 + so2 → hcl + h2so4

Answer 1

The balanced equation for the above reaction is 
H₂O + Cl₂ + SO₂ ---> HCl + H₂SO₄
redox reactions are made of reduction and oxidation reactions.
reduction is when the oxidation state of the species reduces and in oxidation the oxidation state increases in the species.
in the above reaction,
oxidation state of Cl in Cl₂ is 0
and oxidation state of Cl in HCl is -1
theres a reduction in oxidation number of Cl from 0 to -1.
therefore Cl is reduced in the reaction 

Answer 2

Explanation:

Reduction means the gain of electrons or decrease in number of oxidation state of an atom.

For example, $H_{2}O + Cl_{2} + SO_{2} \rightarrow HCl + H_{2}SO_{4}$

Reduction half reaction: $Cl_{2} + 1e^{-} \rightarrow Cl^{-1}$

Here oxidation state of chlorine changes from 0 to -1, that is, chlorine is reducing in this chemical reaction.