Answer:
A horizontal line; a melting ice cube.
Explanation:
A constant interval appears as a horizontal line. The graph in the interval has a slope m = 0.
Here's an example of a constant interval.
You take an ice cube from the freezer and place it in a container on the counter.
The temperature of the ice cube is -5 °C. The ice cube will warm to 0 °C, and then it will start to melt. While it is melting, the temperature is constant at 0 °C.
When the ice has completely melted, the water will start warming to room temperature.
A plot of temperature vs. time would look something like the graph below.
The interval AB, where the ice is melting, is a constant interval.