Which surface would be most suitable for dew formation?

Correct answer: Grass cooled at night

Direct answer: C, grass cooled at night. Dew forms when a surface loses heat and becomes cool enough for nearby moist air to reach its dew point. Water vapour then condenses into liquid droplets on that surface. Grass is a common example because individual blades are exposed to the night sky and can cool efficiently, allowing dew to collect. C is correct because it combines a suitable surface with the necessary nighttime cooling. A is less suitable as stated because dry sand heated during the day is warm and does not immediately provide the cool surface required for dew. B is wrong because burning rock is far too hot for dew to remain as liquid droplets. D is wrong for the same reason: very hot metal prevents condensation on its surface. Dew also depends on moisture and relatively calm conditions, so cooling alone is not an absolute guarantee. Memory cue: look for a cool surface after a moist night; grass is the classic dew example.