The heating curve of an ice-water mixture that is slowly heated to 125? ? c contains three sloped and two level portions. what do the two level portions in the graph represent? a) heating b) deposition c) sublimation d) phase changes

I believe the answer is D. phase changes. The two level portions represents change of state that does not involve change in temperature (at a constant temperature). The first level represents a change of solid to liquid;p where the ice melts and becomes water by gaining the latent heat of fusion, while the other level represents a change of state from liquid to gas; the water changes to steam (water vapor) by gaining the latent heat of vaporization.

Answer

D)phase changes

Explanation
Ice is water in solid state. It is in this state when the temperature is below 0°C. When the ice is heated its state will change from solid to liquid then to gas.
When a substance is changing state, it does so without change in temperature. At this point the curve levels because there is no change of temperature. 
The slopes represent change in temperature with time. 
The levels represent the change phases. Which are from solid to liquid and the other one from liquid to gas.