Explanation:

All codes are correct.

• Recently, in the First International Conference on Steel Slag Road jointly organised by CSIR-CRRI and PHDCCI at New-Delhi, the Guidelines was released for Utilisation and Processing of Steel Slag as Processed Steel Slag Aggregates in Road Construction.

About Steel Slag

• Steel slag is a byproduct generated during the steel-making process, specifically in the production of iron and steel. 

• Formation: Steel slag forms when impurities in the molten steel are oxidized and separated from the molten metal. It can include elements like silicon, manganese, phosphorus, and sulfur.

• Composition: The composition of steel slag varies depending on the process and type of steel production. It generally consists of lime (CaO), silica (SiO2), alumina (Al2O3), and magnesia (MgO) in varying proportions.

• Applications:  Road base course material, Raw material for rock wool, Calcium silicate fertiliser, Fine aggregate for concrete, Soil improvement, raw material for cement etc. 

Hence option 4th is correct.