The correct option regarding steel is option 'D', which states that all of the following statements are correct regarding steel:

a) Carbon content in any form of steel is less than 1.25%.
b) Silicon content should be less than 0.5%.
c) Sulphur content should be less than 0.1%.

Here is a detailed explanation of each statement:

a) Carbon content in any form of steel is less than 1.25%:
- Steel is an alloy primarily made of iron and carbon, with the addition of other elements depending on the desired properties.
- The carbon content in steel plays a crucial role in determining its strength, hardness, and other mechanical properties.
- Generally, carbon content in steel can vary from very low to high percentages, depending on the type of steel being produced.
- However, for most common structural steels, the carbon content is usually less than 1.25%.
- Higher carbon content can lead to increased hardness and strength but may also reduce ductility and weldability.

b) Silicon content should be less than 0.5%:
- Silicon is another element commonly found in steel, typically introduced during the steelmaking process.
- Silicon is often added to improve certain properties of steel, such as its strength and resistance to corrosion.
- However, excessive silicon content can negatively affect the steel's ductility and weldability.
- Therefore, to maintain the desired balance of properties, the silicon content in steel is usually kept below 0.5%.

c) Sulphur content should be less than 0.1%:
- Sulphur is an impurity that can be present in the raw materials used for steel production.
- High sulphur content in steel can lead to brittleness and reduced toughness.
- Additionally, sulphur can also cause problems during the machining process, resulting in poor surface finish and tool wear.
- To ensure good quality steel with adequate mechanical properties, the sulphur content is typically controlled and kept below 0.1%.

In summary, all of the statements mentioned in option 'D' are correct regarding steel. The carbon content in steel is generally less than 1.25%, silicon content is usually less than 0.5%, and sulphur content is typically less than 0.1%. These limits help maintain the desired properties and quality of steel.