From the above options, the best Thermal insulator will be a plastic cup.
Option (b);
Explanation
Thermal insulators resist conducting energy or reduction of heat transfer when objects come across in contact with radiation or higher heat object. To conduct energy, we need metal or those materials which contain free electrons in it to move from one metal to another.
Plastic doesn’t have metallic character so it will resist the heat to some threshold. Mostly, this material is used in separating two current-conducting materials. There are many other examples of thermal insulator such as rubber, fabrics, paper, wood, wool.
