The correct answer is: D. Furrow method
Furrow irrigation is a method of irrigation in which water is applied to the soil surface in a series of furrows. The water flows along the furrows and seeps into the soil, irrigating the crops.
Furrow irrigation is a simple and effective method of irrigation that is well-suited for undulating sandy fields. The furrows can be easily made on the sloping land, and the water flows evenly along the furrows, ensuring that all of the crops are evenly irrigated.
The other options are not as well-suited for undulating sandy fields. Sprinkler irrigation is a method of irrigation in which water is sprayed onto the crops from above. This method is not effective on sloping land, as the water will tend to run off the land instead of being absorbed by the soil. Free flooding is a method of irrigation in which water is allowed to flow over the surface of the land. This method is also not effective on sloping land, as the water will tend to run off the land instead of being absorbed by the soil. Check method is a method of irrigation in which water is applied to the soil surface in a series of small basins. This method is not effective on sandy soils, as the water will tend to drain through the soil too quickly.
Therefore, the best method of irrigation for standing crops in undulating sandy fields is furrow irrigation.