excel says that 80% of SO2 generated is due to combustion of fossil fuels, and that motor vehicles contribute to only 1% emission
so my best pick would either a or b, and since excel didnt mention any statistics on SO2's emmision due to smelting, then id pick b) combustion of fossil fuels
but then again, you cant trust one sources info, and the stats are just for SO2 anyway..
hope that helps!