ANS:D - all 'a', 'b' & 'c'.

Silicon makes steel usable in almost all magnetic circuits where alternating current is used: Silicon is added to steel to enhance its magnetic properties, particularly its permeability and electrical conductivity. This makes silicon steel ideal for use in magnetic circuits where alternating current (AC) is present, such as transformers, electric motors, generators, and inductors. Silicon steel exhibits low core losses and high magnetic saturation, making it efficient for AC applications.

1. Silicon increases steel's electrical resistivity and decreases hysteresis loss: The addition of silicon to steel increases its electrical resistivity, which reduces eddy current losses in electrical devices. Additionally, silicon helps to minimize hysteresis loss by promoting the formation of smaller and more uniformly distributed magnetic domains within the steel. This results in lower energy losses due to magnetization and demagnetization cycles, leading to improved efficiency in electrical devices.
2. Silicon content in transformers and motor armatures: In the context of transformers and motor armatures, silicon is typically added to steel in specific percentages. For transformers, silicon steel may contain up to 5% silicon, while for motor armatures, the silicon content is usually lower, around 4%. These silicon levels are optimized to achieve the desired magnetic and electrical properties required for efficient operation in each application.

In summary, the addition of silicon to steel contributes to its suitability for AC magnetic circuits, improves its electrical properties by increasing resistivity and reducing losses, and is carefully controlled in transformers and motor armatures to meet specific performance requirements. Therefore, all the statements (a), (b), and (c) are correct.