The answer is B. The carbon dioxide level in the city will be higher because the exhaust gases from vehicles release carbon dioxide and there are not many trees to utilize it.
There are more plants in the garden than on the city road. It is known that plants absorb carbon dioxide and together with water in the process of photosynthesis produce their own food and release oxygen. Vehicles release carbon dioxide to the atmosphere. So, if there are enough plants, the amount of carbon dioxide can be absorbed by the plants. Since there are not many plants on the city road, it is expected there will be higher level of carbon dioxide than in the plant-rich garden.