The correct answer is A. Nitrogen.
Nitrogen is the most important nutrient for wheat cultivation. It is essential for the production of chlorophyll, which is necessary for photosynthesis. Nitrogen also helps to increase the size and yield of wheat crops.
Potassium is also an important nutrient for wheat cultivation. It helps to improve the quality of the wheat grain and to increase the resistance of the plant to disease.
Copper and iron are essential nutrients for all plants, but they are not as important for wheat cultivation as nitrogen and potassium. Copper is involved in the production of chlorophyll and iron is involved in the transport of oxygen.
In conclusion, the correct answer is A. Nitrogen.