The correct answer is The water vapour present in the air condenses on the surface of the bottle.

• Moisture in the air called "humidity".
• The amount of moisture that can be in the air depends on the temperature of the air.
• The warmer the air is, the more water it can hold in it.
• When the air comes up against the cold outside of a cold bottle, the air right next to the bottle gets cold.
• It gets so cold that it can't hold as much water anymore and some of the water "condenses" or comes out of the air onto the bottle.
• That cold air next to the bottle is heavier than the air around it (hot air rises, cold air sinks) so the colder air with some water out of it sinks, and fresh water-filled air comes in to get cooled and leave some more condensation.
• This process is similar to what causes clouds to form and rain to fall. Moisture in the air is condensed and precipitated because of cooler temperature or similar effect.
• The formation of dew is almost exactly like the formation of the water droplets on the outside of a cold bottle.