Explanation:

• Steel slags can be used in several activities, such as construction and paving, and also in the agricultural sector due to its ability to correct soil acidity, as it contains some nutrients for the plants and also as silicate fertilizer that is capable of providing silicon to the plants. Steel-slag can be used to produce energy-saving cement by co-grinding with OPC clinker and blastfurnace slag.
• Slag-based fertilizers applied in Si-deficient paddy soil has shown to improve rice growth, productivity, and brown spot resistance. The application of steel slag in the soil increases the concentration of Si in the rice straw and promotes a higher yield of grains.

Hence option 4th is correct.

For More: https://www.sciencedirect.com/science/article/pii/S2090447912000573#:~