Answer: the horizontal line in a heating curve represents a change in state (second option or letter B)
Step-by-step explanation:
The question requires us to identify what a horizontal line represents in a heating curve.
A heating curve in chemistry corresponds to a graph where the temperature is plotted on y axis agains the heat absorbed (or time) on the x axis, and it represents the behavior of a substance in terms of the states of matter when the temperature and heat (over time) changes.
For example, we can represent the heating curve of water as in the following image:
Note that the temperature where the line stays horizontal corresponds to the change of state of water (melting at 0°C and boiling at 100°C), while the other regions of the graph corresponds to where the water in its solid, liquid or gas form is absorbing heat and increasing the temperature.
Considering the information above, we can say that the horizontal line in a heating curve represents a change in state (second option or letter B).