ANS:D - increases the fluidity of molten metal.

Increasing the fluidity of molten metal refers to its ability to flow more easily during casting processes. When molten metal has higher fluidity, it can fill intricate molds and cavities more effectively, resulting in the production of complex shapes with greater precision. The addition of silicon to cast iron increases its fluidity through several mechanisms:

1. Reduced Melting Point: Silicon lowers the melting point of the iron-carbon alloy, allowing it to remain molten at lower temperatures. This reduction in melting point enhances the flowability of the molten metal.
2. Surface Tension Reduction: Silicon reduces the surface tension of the molten metal, which decreases the resistance to flow. This reduction in surface tension allows the molten metal to spread more easily and fill the mold cavity efficiently.
3. Improved Wetting: Silicon promotes better wetting of the mold surfaces by the molten metal. This means that the molten metal can spread out and adhere to the mold surfaces more effectively, facilitating the casting process.