Option-1: Both the statements are individually true and Statement II is the correct explanation of Statement I.

This option is correct. Statement I states that a thermos flask is made of double-walled glass bottles. This is true as a thermos flask consists of two layers of glass with a vacuum between them to minimize heat transfer. Statement II states that metals are good conductors while gas and air are poor conductors of heat. This is also true as metals have free electrons that can easily transfer heat energy, while gases and air have fewer particles and therefore transfer heat more slowly. Statement II is the correct explanation of Statement I because the double-walled glass bottles act as insulators, with the vacuum between the layers of glass providing insulation by preventing heat transfer through conduction or convection. Therefore, the combination of a thermos flask being made of double-walled glass bottles and the properties of metals, gas, and air correctly explains why a thermos flask is effective at maintaining the temperature of its contents.