Final answer:
A piece of black plastic is most likely to become the hottest when left in the sun due to its color and properties, which lead to it being an effective absorber and radiator of heat.
Step-by-step explanation:
All objects absorb and emit electromagnetic radiation, but this rate of heat transfer varies based on the color of the object. A black object is the most effective at both absorbing and radiating heat. In comparison, a white object reflects most of the radiation and is therefore a poor absorber and radiator. Taking into account thermal conductivity, metals like aluminum are good conductors of heat, while materials such as wood and plastic are poor conductors. Therefore, of the materials listed, a piece of black plastic would probably become the hottest if left in the sun for several hours.