The correct classification for silicon can be concluded from option D:
D: Silicon is a metalloid and has properties of both metals and nonmetals - it appears lustrous, but is brittle and a poor conductor of heat and electricity.
Silicon is classified as a metalloid because it exhibits characteristics of both metals (such as having a shiny appearance) and nonmetals (such as being brittle and not a good conductor of electricity under certain conditions). It does not fall into the categories outlined in options A, B, or C, as those descriptions inaccurately classify silicon.