The correct option is
C Highly industrialized regions of the world
Nearly two-thirds of annual SO
2 emissions and just over one-fifth of NO
x emissions are produced by electric utility plants that burn fossil fuels. Transportation sources (e.g., cars, trucks and other vehicles) account for more than half of the total NO
x emissions. Ammonia emissions arise largely out of livestock waste and fertilised soil. Industrial combustion and industrial processes are also major sources of such emissions. Therefore acid rain usually occurs in highly industrialised areas.