Photosynthesis is important for the plants to make sugar (broken from glucose) available for the plant and to release oxygen. Therefore, a. and d. are incorrect, because sugar is a product of the reaction, not something that's needed for the process to occur.
Nitrogen is available in the soil and it is very important to plants, but it doesn't play any part in photosynthesis. Therefore, c is incorrect as well.
In order to obtain glucose and oxygen, the plants needs the reactants water (from soil, rain, etc) and carbon dioxide (mainly from the air). Therefore, the correct answer is b. water and carbon dioxide.