Answer: The correct answer is deposition.
Step-by-step explanation:
Deposition: It is a process conversion of gaseous state directly to the solid state by escaping the liquid state.
Gas → Solid (Deposition)
The cause of frost on the grass in the autumn is ,when the temperature of the air near to ground decreases down to the temperature lower than freezing point of the water. This decrease in temperature of the air leads to formation of ice crystals from the water vapors present in the air on the grass.
Water vapor → Ice crystals (Deposition)
Hence the correct answer is option(a).