Which of the following reactions is a redox reaction?
(a).`CuSO_(4)+4NH_(3) to [Cu(NH_(3))_(4)]SO_(4)`
(b).`Na_(2)SO_(4)+BaCl_(2) to BaSO_(4)+2NaCl`
(c ).`SO_(2)+H_(2)O to H_(2)SO_(4)`
(d).`2CuSO_(4)+4KI to Cu_(2)I_(2)+2K_(2)SO_(4)+I_(2)`

Explanation

The reaction in which change in oxidation number of some of the atoms takes place is termed as a redox reaction.
(a) `overset(+2+6-2)(CuSO_4)+ overset(-3+1)(4NH_(3)) to [overset(+2)(Cu) overset(-3 -1)((NH_(3))_(4)] overset(+6-2)(SO_(4)]`
No change in oxidation number of any of the atoms.
(b) `overset(+1+6-2)(Na_(2)SO_(4))+overset(+2-1)(BaCl_(2)) to overset(+2+6-2)(BaSO_(4))+overset(+1-1)(2NaCl)`
No change in oxidation number of any one of the atoms.
(c) `overset(+4-2)(SO_(2))+overset(+1-2)(H_(2)O) to overset(+1+4-2)(H_(2)SO_(3))`
No change in oxidation number of any one of the atoms.
(d) `overset(+2+6-2)(2CuSO_(4))+overset(+1-1)(4KI) to overset(+1-1)(Cu_(2)I_(2))+overset(+1+6-2)(2K_(2)SO_(4))+overset(0)(I_(2))`
Oxidation number of Cu decreases from +2 to +1 and oxidation number of iodine increses from -1 to 0.
Thus, out of the above four reaction, the reaction (d) is a redox reaction.