The correct answer is - B. fuel burning in vehicles.
The sulfur oxides are one of the major polluters in the atmosphere. Also, they are one of the main polluters that cause the appearance of acid rains all across the globe. The main source of this polluters in the atmosphere is the combustion of the fossil fuels in the vehicles, and unfortunately there's a constant grow in vehicles that rely on fossil fuels for functioning all over the globe. The biggest nature cause for this polluters to end up in the atmosphere are the volcanoes, but compared to the vehicles it is much less.
The acid rains are a big concern as they are very damaging to all the living organisms, making people and animals develop respiratory problems, and destroying the vegetation (as witnessed with the dying out of lot of forests in Europe). They also cause a lot of damage to the infrastructure as they are very corrosive.