S has many oxidation states including -2,0,2,4,6...
in this particular reaction the oxidation state of S is "+2"
a decrease in oxidation state - reduction
an increase in oxidation state - oxidation
1.) S + O2 -> SO2
here at the left handside, as sulphur exists itself (not compound), the oxidation state is 0
at the right hand sidde it forms a compound and gains an oxidation state of +2 = this is oxidation
whereas O has an oxidation state of "0" in the left hand side and decreases to -4 in the right hand side
this is reduction
therefore its redox