Explanation:

- Split application of fertilizer: Dividing the total amount of nitrogen fertilizer into multiple applications throughout the growing season allows plants to absorb nutrients more effectively, reducing waste and improving nitrogen use efficiency.

- Use of nitrification inhibitors: These compounds slow down the conversion of ammonium to nitrate. This helps in reducing nitrogen loss from leaching and denitrification, making more nitrogen available to the plants.

- Use of slow-release fertilizers: These fertilizers release nitrogen gradually, matching the nutrient release with the plant's growth needs, thus enhancing nitrogen use efficiency and reducing losses.

- All the above: Each option listed contributes significantly to improving nitrogen use efficiency in different ways.

Answer: Option 4 - All the above.