Final answer:
Air is the best insulator among the options given, whereas silver, copper, and iron are metals with high thermal conductivity making them poor insulators.
Step-by-step explanation:
Of the options given, the best insulator is air. Silver, copper, and iron are all metals known for their high thermal conductivity, which makes them good conductors of heat and not good insulators. This is due to the density of free electrons that can transport thermal energy efficiently.
For insulation purposes, materials with low thermal conductivity are desirable. Materials such as fiberglass and Styrofoam are often used as insulators because they incorporate small pockets of air. Air's poor thermal conductivity makes it an excellent insulator, hence, in a comparison between metals and air, air is the superior insulator.