Answer: Option C. -> Ice at $0^{\circ }C$
:
C

Ice at $0^{\circ }C$ can be more effective in cooling because of the phenomenon called latent heat of fusion. Latent heat of fusion of ice is the heat absorbed by ice to change its state to water at $0^{\circ }C$ . So, ice at $0^{\circ }C$  can extract more heat from another substance and give better cooling.

Answer: Option C. -> Humidity
:
A, B, and C
Evaporation is a process where liquid converts to gases. It is a surface phenomenon.
The factors affecting evaporation are:
$\bullet$ Surface area - The increase in surface area increases the rate of evapoartion as it is a surface phenomena.
$\bullet$ Temperature - The movement of the particles increases which helps them to escape from the surface easier and faster.
$\bullet$ Humidity - If the humidity (water vapour) in the air is more, it is difficult for the other water particles from any surface to enter into the atmosphere. Thus, it decreases the rate of evaporation.
The density of the substance has no role in the process of evaporation.

Answer: Option A. -> diffusion 
:
A
Diffusion is a process by which particles of matter intermix with each other, spontaneously. The fragrance from one corner of the room spreads throughout the room as the particles constituting the fragrance intermixes with the particles of air in the room.

Answer: Option B. -> Ice converts into water by absorbing heat.
:
B

Ice, being a solid, comprises of very closely packed molecules which possess less kinetic energy. To convert it into the liquid state, heat energy has to be supplied to overcome the forces of attraction between the particles. These particles absorb the heat energy, break the force of attraction and turn into liquid state.