What variations are there on trigonal planar geometry based on replacing bonds with lone pairs

Which type of reaction always has an element and a compound as reactants

The answer for #3 is c but i don't know why

Silver tarnishes as silver metal reacts with hydrogen sulfide, h2s, in the air. in this reaction, dark silver sulfide, au2s, covers the surface of silver. when silver is polished, this coating of silver sulfide can be removed from the surface. this makes the silver shiny again. enter the coefficients that balance the tarnishing reaction equation. (type 1 for no coefficient.)