The correct answer is D. Si.
Metallic impurities are elements that are not part of the main composition of glass, but are present in small amounts. They can be introduced into the glass during the manufacturing process, or they can come from the raw materials used to make the glass.
The most common metallic impurities in glass are iron, copper, and manganese. These elements can cause the glass to absorb light in certain wavelengths, which can result in undesirable color changes.
Silica (SiO2) is the main component of glass, and it is not a metallic impurity. It is a non-metallic oxide that is found in sand. Silica is what gives glass its strength and durability.
The other options are all metallic impurities that can be found in glass. Iron (Fe2+ and Fe3+) can cause the glass to absorb light in the blue and green wavelengths, which can result in a yellow or brown color. Copper (Cu) can cause the glass to absorb light in the red and orange wavelengths, which can result in a green or blue color. Manganese (Mn) can cause the glass to absorb light in the violet and ultraviolet wavelengths, which can result in a yellow or brown color.