When a substance changes its state from solid to liquid
A. The temperature remains constant
B. There is a constant rise in temperature
C. There is a constant drop in temperature
D. The temperature fluctuates

Hint: Temperature is the measure of the average of vibrations of molecules. When a substance changes its state, all the heat energy is used up in change of state from solid to liquid instead of increasing the vibrations of its molecules. Thus temperature will remain constant while changing state.

Complete answer:
What is temperature? Temperature is a measure of average kinetic energy of all particles in a material. When heat is added to the material, it is used up to increase the kinetic energy of the particles and thus the average also increases which as a result shows up in increase in the temperature of the material.
Now our question is what happens when the same material changes its state. We know that at different temperature and environmental conditions, water has three states of matter, namely the popular one, water for its liquid state, ice for its solid state and water vapour for the gaseous state. When a material is going through change of state, here solid to liquid, due to addition of heat, it is consumed by the particles to overcome the interparticle attractive forces that have kept them bonded in the particular state and there is no net increase in average kinetic energy. This heat energy is called the latent of fusion. Thus the temperature remains constant. So option A is the correct one for the above question.

Note: At particular pressure conditions, this constant temperature is known as the melting point. As the environmental conditions change, the melting point changes for the same substance. An interesting example is like we have a water cycle here on earth, we have observed methane cycles on Saturn’s largest moon Titan with methane existing in three states vapour, liquid and solid. The pressure conditions determine state of matter.