Short Answer:
C)
Long Answer:
A- When a solid is heated, it melts and becomes a liquid. (Another older term is Fusion.) An example of this is ice to water when you leave it outside the freezer.
B- When a liquid becomes a gas, it undergoes a process called boiling. Example: When you boil water, it becomes steam.
C- It is called deposition. Example: How snow forms in clouds.
