The correct answer is - China has fewer pollution controls.
China is a country that has an enormous industry which requires lot of energy for its functioning. Most of the energy is gained from the burning of the fossil fuels, mostly the coal, which is one of the biggest polluters.
Japan, on the other side, is also a country with highly developed industry, but unlike China, it is much more versatile when it comes to the usage of energy, as it uses energy from multiple different sources, some of which are very ecological too.
The biggest problem and difference between these two countries, and the problem with the pollution is the control of the industrial facilities. While Japan is very strict when it comes to pollution control, and keeps everyone in order, China is the other way around. In China, the pollution controls exist just formally, but in practice they do little to nothing to reduce or stop the pollution.