The best example of a desiccant is option 1, silica gel. A desiccant is a substance that is used to absorb moisture or water vapor from its surroundings. Silica gel is a porous, synthetic substance made from silicon dioxide. It has a high affinity for water molecules, which allows it to effectively absorb moisture from the air.

Option 2, polystyrene, is a type of plastic and does not possess any desiccant properties. It is not able to absorb moisture or water vapor.

Option 3, sodium chloride, also known as table salt, is a hygroscopic substance, meaning it can absorb moisture from the air. However, it is not commonly used as a desiccant and has limited effectiveness compared to silica gel.

Option 4, sodium carbonate, is not typically used as a desiccant. Although it can absorb moisture in certain applications, it is not as effective as silica gel.

In this case, option 1, silica gel, is the best example of a desiccant as it is specifically designed and widely used for its moisture-absorbing properties.