The correct answer is: A. Crop rotation.
Crop rotation is a farming practice in which different types of crops are planted in the same field in a planned sequence. This helps to improve soil fertility by preventing the build-up of pests and diseases, and by replenishing nutrients in the soil.
Improved methods of agriculture, such as using cover crops and no-till farming, can also help to improve soil fertility. However, these methods are not as effective as crop rotation in preventing the build-up of pests and diseases.
Using new seed varieties can also help to improve soil fertility, but only if the new varieties are resistant to pests and diseases. If the new varieties are not resistant, they may actually damage the soil.
Irrigation can help to improve soil fertility by providing water to crops during dry periods. However, irrigation can also lead to soil erosion and salinization if it is not done properly.
Therefore, crop rotation is the most effective way to improve soil fertility.