Answer:
a. friction
Explanation:
What is a common unbalanced force acting in motion? Common forces that are often unbalanced include the force of gravity and applied forces. When these forces are unbalanced, objects accelerate, change their position and find new configurations for which all forces are again balanced.
Friction is a kind of force.
A heater letting of heat is an example of release of energy. Please comment if you have any questions about my answer.<span />
When it comes to physical changes like phase changes, there are two types of heat energy: sensible heat and latent heat. Sensible heat is the heat absorbed/released when you heat the substance but it doesn't change phase. An example would be heating lukewarm water. The substance is liquid all throughout. Latent heat, on the other hand, is the heat absorbed/released when there is a phase change. An example would be boiling water, because it changes liquid to vapor.
Hence, for freezing liquid, you use the latent heat, specifically the heat of fusion. The answer should be
2.5 g * (1 mol/18.02 g) * 6.03 kJ/mol = 0.84 kJ/mol
The answer is not in the choices. You only use Hvap if you boil water.
